A vital piece of information that could save a lot of time and trouble is that you don’t have to take any dice into account for fights between DM controlled creatures. In Critical Role when the plank king grabs and kills Captain Avantika, no dice are rolled. This is between two NPC’s and therefore the DM simply says what happens. You wouldn’t roll dice for a similar battle hundreds of miles away from the PC’s why do the same here, if the players just watch, describe what you think should happen, in rules terms maybe subtract a quarter or so of its health. If the players attack the dragon with the army, then have the army deal a fairly sizeable amount of damage on initiative count 20. The most important thing to remember is that the rules are designed purely as a crutch to help the DM’s story run smoothly, not the other way around. Hopefully this is helpful!
Ok first of all I understand your players disappointment, they have come up with an approach that seems clever and novel and want to try it. But I also get how for you this is frustrating. So some things to consider.
First of all dragons are very intelligent, this one is not going to just engage an army in open battle on the ground where it can be killed. It’s lair should be guarded and protected and hard to get to, not the kind of place an army can easily advance on either meaning the advantage of numbers will be negated. Or it will fly away, burying its horde, to return tomorrow or the next day.
Normal citizens are not heroes; there is a reason they needed the heros in the first place, even soldiers will not be as resilient as the heros, imagine the damage a dragon breath as it flies across the battlefield will do to massed troops, the morale impact that will have.
But you want to give the players a sense that they gain some benefit from that army. (How big an army can be roused, remember normal citizens won’t take up arms and the towns need to keep some protection so you are unlikely to be taking thousands). So have the dragon have minions and creatures that will fight for it. Have those forces come out to meet the army but the benefit to the players is that none of these forces are between them and the dragon now. The dragon won’t be coming out of its lair and the army can’t easily get up to its lair (and if they do will be a big mass of troops that can be knocked off the cliff). Don’t tell the players this, explain the route, maybe have NPCs explain the risks of the players insist then have there forces hit by dragon breath on the narrow paths, or the dragon hurl boulders down,
The army could be a great distraction allowing the players to sneak on the dragon or a hindrance depending how they use it, but it never need actually engage because, a 20 HP soldier with low AC and no shelling potion will die very quickly.
A vital piece of information that could save a lot of time and trouble is that you don’t have to take any dice into account for fights between DM controlled creatures. In Critical Role when the plank king grabs and kills Captain Avantika, no dice are rolled. This is between two NPC’s and therefore the DM simply says what happens. You wouldn’t roll dice for a similar battle hundreds of miles away from the PC’s why do the same here, if the players just watch, describe what you think should happen, in rules terms maybe subtract a quarter or so of its health. If the players attack the dragon with the army, then have the army deal a fairly sizeable amount of damage on initiative count 20. The most important thing to remember is that the rules are designed purely as a crutch to help the DM’s story run smoothly, not the other way around. Hopefully this is helpful!
This is not the best advice, that moment with avantika was not the crux battle, in fact Matt brought her back later so the party never felt robbed of that fight. Matt did indeed roll dice behind the screen, you might not have seen it but he did it. But he also weighed up the situation. This is very different to that and should not just be described to the players as spectators, they should be in the middle of the battle fighting with the army, watching soldiers who are far weaker then them die, a more accurate description using critical roll would be the attack of the chroma conclave on Emon, Matt described the deaths of loads of people, including soldiers, while also having the players in the middle,
For some ideas on ways you could run this forward this episode to 3:30 and watch till the end. It is a great example of how you can create that sense of a large battle without rolling hundreds of dice.
And in that one Matt actually has the party fight a black dragon with a small army, watch from 2:35ish. To get to the action (trap was set, many people hidden, lots of NPCs).
There are two approaches you can take with the party.
First explain the mechanics of D&D 5E, how critical hits work and explain to them that if they have 100 archers in each of the 5 humanoid type armies, that's 500 bows, of which 5 percent will hit as critical hits meaning the dragon will take 25 critical hits per turn before it gets into fear range. So, lets say they are all Dex 10, so (2d8) * 25 * .5 = 200 damage done on average in the first turn, not counting 19's that will kill the dragon in one turn. If it is an adult red dragon, it would have 56 hit points. Now ask the party, if the dragon knew that was coming, as old as it was, would it simply stand up to let you players roll the critical hits and die or would he simply flee and you'll never get to fight him with your army in the first place? Furthermore, is that how you want to go for the final battle? If the players say yes they want that, I'd give them exactly this fight they win, I'd say thank you for them playing and drop them. I truly hate players who behave like this frankly, they don't want to play the game they want to have no chance at death, they just want to win no matter what. They could also try sneaking in, poisoning the dragon or stealing something of worth rather than throwing overwhelming numbers.
Second explain to compensate for this vastly overwhelming force, the dragons lair will be quite upgraded. He will have a decent size defense force and a very good chance of winning, otherwise if he saw that force, he'd collapse his lair and fly off and choose not to fight. Explain to the players each one will have to roll for each 100 and keep track of their rolls. You expect them to pay attention during the session and deal with the 10 to 15 minute turns until the army is worn down. They want that mess, explain how its going to play out. If you want to reduce it down to smaller numbers you can do that as well, but if its large town, they are going to balk from the sounds of the players you have now. Even then, it doesn't matter the town owes them, its highly unlikely that CR 0 humanoids will go into a dragons lair to fight, they aren't heroes. The party is heroes, heroes go out and slays dragons, not a conscript with a chain shirt, a mace and a light crossbow. I always set up dragon fights to be minimum a Hard fight by CR if the party has went through the lair with the daily XP budget taken into account. So if they are a CR 80 group with that many men, they'll be fighting a CR 100 with his allies.
Frankly no town would risk pissing off an adult red dragon and have it be traced back to them. At best give them maybe one representative from each village and give the dragon a good chance of recognizing what village they are from. If the dragon can escape, have the dragon attack the town it recognized and lay waste. Perhaps the players are in town to help or they realized what would happen. Dragons have escape plans, especially adult dragons, they don't grow that old if they are stupid.
The dragon also doesn't have to be alone either. The dragon could be a female red with about 5 adult red male dragons fighting to mate with her as the army approaches. If they have 500 archers, its now a fair fight with the males going suicidal to prove they are the strongest to mate with her. Now they have to figure out how to deal with the horde of horny red dragons and an Army that will most likely be in rout plus the female red dragon will know the party tried to kill her and the audacity while she was in her mating cycle, something must be done about the party now.
This is trying to stop the players out of game, personally I dislike doing this because you openly remove player agency, yea as DMs we secretly remove agency all the time, we do things based on what the players have done, tweak environments and encounters or mix things up just cos we can (personally never in combat but sometimes right before I will make tweaks). But telling your players flat you, you can’t do that, it isn’t logical, is bad. You can however have your NPCs make some valid points like, what if the dragon swoops from the air, or, where is its lair, how will an army climb up that narrow 1 person wide mountain pass, or, won’t the dragon see us marching on it. You can have a a grizzled veteran talk about a regiment that got burnt to a crisp before ether could let off an arrow, or have a soldier ask the hero’s if they can teach him how to cast a spell, or use that funky fighter ability so he might survive.
Have the characters realise and think about things and, if they insist anyway, then don’t have the dragon play fair, boulders dropped from on high, breath attacks as it swoops over the army and away, it’s minions attacking the army. Find ways in game ti encourage the characters to realise that the best approach is a sneaky approach, let them use the army as a distraction, maybe they tell them to scatter and run if they are the dragon and make it look like the dragon scared them away meanwhile party get to the lair un impeded and that is the benifit of the army, no pesky encounters using up resources on the way up to its lair. But never ever tell players, because you thought of X I am now going to make things harder, I have seen that approach actually drive players away from a DMs table because they thought what’s the point.
While you do recruit an army in Dragon Age: Origins, the point is not to use it to fight the dragon (end boss). The army is recruited to fight the army that the dragon has so that the heroes can reach the dragon. And the reason that you're able to recruit that army in the game is because the main character belongs to an organization with the authority to do so under the circumstances that occur in the game. From what I can tell, the PCs in this scenario lack that kind of leverage. The NPCs might be grateful for what the PCs did for them, but that doesn't compel them to become cannon fodder for the PCs. Have them offer up some alternative form of aid.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
I’m not suggesting the players are spectators and nothing more, I’m simply trying to explain how to save a lot of time and likely stress adding the army to what will already be a long and complex combat to run. Further down I do give some advice on how to handle the players attacking simultaneously with their forces. On the other hand you are right, and I did definitely overlook some points. Thanks for the feedback.
BTW - I *hate* current video games, and have never played anything since Okami with the PS2. So I don't know everything about Dragon Age, just watched YouTube videos on it. I guess an army fights the dragon with you.
If you hate video games that much, why are you running your dragon like a video game?
Look, I get it. I don't play current video games either. They're predictable and boring. Better graphics has always been easier than better AI, and so you end up with beautifully rendered monsters who can't think beyond the end of their claws, and can't adapt tactics from their programmed script.
It doesn't mean the games are worthless. They have their strengths and weaknesses, just like anything else. If you want an immersive audio/visual experience, play a video game, I can't begin to compete; but if you want to wargame out a battle with a devious dragon, come to my tabletop. Bring your armies, too. Because that dragon does not live in a block of my memory waiting to be accessed whenever there is a relevant call for the data. It actively schemes and works against its enemies, spying, probing, and adapting to circumstances. Also because I don't need to have a script set out in advance for everything the players might or might not do, but instead I get to see what they're doing and focus my creative energy on responding to what they're doing with all the resources of a supernatural apex predator.
As others have illustrated here, low level minions are not threats to a dragon. Also, look at DMG p.92-4 and be real with your players about what they're asking for, which is essentially a suicide mission with expected casualty rates over 90%. Do the circumstances of their alliances really allow them to ask for such a sacrifice? And speaking of conscripting sacrificial minions, don't be shy about giving this dragon an army of followers too.
If you do go the route of large scale battle, yes see the mob attack rules (DMG p. 250) and handle things narratively that don't involve the players' rolls directly as others have suggested (especially if the dragon has an army too), but also look around to other systems to draw inspiration for running large scale battles. I personally run D&D games out of nostalgia at this point (ok, also WotC paroled themselves from the dungeon of my derision with 5e) but I have come to prefer another system, especially for combat focused games. I won't advocate for them by name here, but they use a system for large scale battles where each side's strength is represented by tokens, and then opposing commanders make opposed rolls which may result in the loss of tokens, and players who are not actively involved in a command role may make individual rolls to aid their side. Back to the WotC line, I believe there were rules in one of the 3e Combat & Tactics or Heros of Battle expansions that handled this too. But all of that assumes that you do in fact want to battle army vs. army, which again there is no reason to do if you don't want to do it.
Give your dragon some respect. Maybe extra HP or abilities if you want to (remember: expecting a dragon or any other monster to be exactly as represented in the MM is like expecting a human adventurer to have all 11s and 10s for ability scores), but give credit to an adult dragon's full range of abilities and resources. Your dragon wasn't programmed into existence to wait timelessly atop its hoard until the players and their armies walk into its lair. And the only way the PCs wishlist is lethal to it is if you let it sit there like a giant rock waiting for someone with a big enough lever to come along.
In short: this is not a time for "you can't do that"; this is a time for "are you sure you want to do that".
If the PC's want to pick a particular fight the challenge is to figure out what the PCs will do in that fight so that it doesn't turn into a simple narration.
I would push back against the suggestions that a dragon would wipe out the army as something to actually do to the PC's/ their army. They've been campaigning for a while. They've racked up accomplishments. Unless you're running a game with a Game of Thrones theme where seasons of exposition are wiped out in a few minutes of CGI and the entire tone shifts on a dime, you kind of have to let them have their army. So the question becomes:
How do you use that army?
A block of yeoman farmers with bows will eventually shoot down a dragon per DND mechanics. At the same time, the dragon will probably wipe ALL of them out with a single breath weapon in the middle of a move while flying. Thus part of the challenge. At the same time a smart dragon is going to use it's lair which is NOT likely to be army friendly and it's not going to fall into any obvious traps where it chases the PC"s out of its lair and into the army itself.
So one angle to take is to go back to your table and say "okay guys, here are some things that the officers in the army are bringing to you as concerns". If you're running an RP heavy game, you can literally create NPC's to represent the elected leaders of these groups (militias often voted for their officers; regular army is paid and thus had paid officers). They can ask the PC"s "what's the plan?" and raise all the questions this thread has so far. What's the plan for the lair? What's our plan to counter it's breath weapon? What's the plan if the dragon raises it's own army?
From a tactical point of view, you can also create layers of the battle(s). Sure the army can drop the dragon in an open fight but first someone has to deal with that necromancer it hired to raise an army to cover it. First someone has to clear the road the army's going to travel for ambushes. When the battle starts some people are going to need keep command (and maybe here you do something like give each PC a command and shift more into a tactical RP with "leadership skill checks" to give them dice to throw to help follow the narrative.
For my money, I would work to find ways to let the army be the thing that beats the dragon because that's the thing the PC's worked for and in many ways "earned".
On a whim I put in a fight into a combat simulator using Goblins as my stand ins for Yoemen fighters and put 80 of them plus 20 goblin bosses (those volunteers who are actually competant warriors, retired adventurers, etc). Up against an adult red dragon here's what it kicked out:
party won 92% of the time barely an inconvenience average casualties when the party wins: 39 die 4% of the time 36 die 4% of the time 81 die 4% of the time 33 die 8% of the time 31 die 4% of the time 30 die 4% of the time 29 die 4% of the time 28 die 4% of the time 27 die 4% of the time 26 die 4% of the time 24 die 12% of the time 70 die 4% of the time 21 die 4% of the time 66 die 8% of the time 18 die 4% of the time 64 die 4% of the time 15 die 4% of the time 8 die 4% of the time 54 die 4% of the time all die 8% of the time average wave reached: 1
04/21/2022 2:51 PMGMT was the time the fight started Per creature report: Breakdown of each creatures actions
simulation ran 25 fights average nbr of rounds: 5 average team damage per fight: 253 average monster dmg per fight: 446 Red Dragon Adult damage: 1939 Red Dragon Adult damage: 1939
I ran the same thing with an ancient red dragon and got:
party won 0% of the time Disaster average casualties when the party wins: all die 100% of the time average wave reached: 1
04/21/2022 3:02 PMGMT was the time the fight started Per creature report: Breakdown of each creatures actions
simulation ran 25 fights average nbr of rounds: 14 average team damage per fight: 169 average monster dmg per fight: 980 Red Dragon Ancient damage: 6695 Red Dragon Ancient damage: 6695
Average HP left after a fight: Goblin: 0/7 Red Dragon Ancient: 376/546 Goblin Boss: 0/21
So if it's just an adult red dragon I think the idea of an army of "cannon fodder" might be unfair. However if it's an ancient, you need to have a plan.
I suggest for the army making it have a total set of hit points that would be visible to the party. 1 hp represents 1 humanoid.
This allows the players to have a sense of how much of their army is left during the fight. Then give the dragon an increased breath attack for strayfing and boost ac up to 21 as it will likely be in flight the whole time. If they bring it out of the sky bring it down to 19 ac as usual.
Should the dragon cripple the army (cripple is to bring to 1/4 total health) then have the army flee. By then the party should be able to finish the dragon themselves
This kind of suprises me I would have thought the dragon would have won more often, was the dragonflying high using hit and run tactics?
And also wiping out large numbers of the army with each breath? The first breath on round 1 (step 27) only hit 5 enemy - the shape of the breath was really weird, so not an accurate simulation at all.
The simulator has the dragon in melee with the enemy in round 3 (step 216) - so it is just doing multiattack in melee, even though it regained its breath weapon at the start of the round; and on the next goblin's turn it does Wing Attack which is only treated as hitting 1 enemy, rather than the 15 which are standing next to it; and it takes Tail attack as LA after the next goblin attack, and nothing thereafter - rather than another wing attack.
I don't think that the simulator works for dragons, or flying creatures.
The general thing that determines army vs BBEG in 5e is number and type of ranged attacker; melee combatants will have limited ability to engage and are easily wiped out by area attacks, but it's quite hard to deal with a bunch of scouts.
I don't think that the simulator works for dragons, or flying creatures.
Yeah it definitely wasn't going for an optional engagement and did rely a lot on "Claw/claw/bite" or "breath weapon" as it's primary choices. Even with disadvantage, a DC19 means that 10% of a force is going to resist and then 10% of the remaining feared force is going to recover every turn. Law of averages says that archers will eventually whittle the dragon down.
Now that said, you can obviously play the dragon smarter but I still think there's room here for the PC's to get what they want (we raised an army to take on this dragon) and still have the engagement be exciting, dynamic and fun.
One thing I'd say should happen is have the Morale mechanics in play. Rather than rolling for every individual creature, I recommend either breaking the army into squads and roll a save per squad or to have a single roll for the entire army to see if morale breaks and it routs. Each round make the save, with increasing penalties as casualties accumulate. Squads/individuals that are affected by the frightening presence would obviously have disadvantage, and add a cumulative +1 to the DC per squad that flees and a cumulative +2 per squad that's killed.
And make sure you tell the players beforehand that these rules will be in play.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Learn to tell your players "no". What level is the party at?
Edit:
Oh, and who says your Dragon has to go by the stat blocs? 90 foot range on a breath weapon for an Ancient Red? Nope: 180 feet sounds reasonable. Darkvision of 120 feet? Nope: 240. Spell casting: Automatic of course.
Nothing like a Ancient Red attacking, at night with cloud cover, with Greater Invisibility cast on itself.
Really disagree with this, players should never be told outright no for something like this, that makes for an awful game where they feel they have no control or say. This is a very valid, very reasonable approach and tactic to take and just batting players wishes out of hand will make them less likely to continue playing.
Instead you should find ways to let them try, give the characters enough information to understand all the many risks involved in this, and if they insist on continuing down this route accept that you may well be killing off tons of NPC's as the dragon whittles away at the army refusing to engage them openly, uses the narrow and hard to get to routes up to it's lair to set traps, and drop boulders on the spread out single file troops etc.
But openly telling players, nope not letting you do that is awful DMing
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A vital piece of information that could save a lot of time and trouble is that you don’t have to take any dice into account for fights between DM controlled creatures. In Critical Role when the plank king grabs and kills Captain Avantika, no dice are rolled. This is between two NPC’s and therefore the DM simply says what happens. You wouldn’t roll dice for a similar battle hundreds of miles away from the PC’s why do the same here, if the players just watch, describe what you think should happen, in rules terms maybe subtract a quarter or so of its health. If the players attack the dragon with the army, then have the army deal a fairly sizeable amount of damage on initiative count 20. The most important thing to remember is that the rules are designed purely as a crutch to help the DM’s story run smoothly, not the other way around. Hopefully this is helpful!
Be Excellent to one another. Rock on dude.
Ok first of all I understand your players disappointment, they have come up with an approach that seems clever and novel and want to try it. But I also get how for you this is frustrating. So some things to consider.
First of all dragons are very intelligent, this one is not going to just engage an army in open battle on the ground where it can be killed. It’s lair should be guarded and protected and hard to get to, not the kind of place an army can easily advance on either meaning the advantage of numbers will be negated. Or it will fly away, burying its horde, to return tomorrow or the next day.
Normal citizens are not heroes; there is a reason they needed the heros in the first place, even soldiers will not be as resilient as the heros, imagine the damage a dragon breath as it flies across the battlefield will do to massed troops, the morale impact that will have.
But you want to give the players a sense that they gain some benefit from that army. (How big an army can be roused, remember normal citizens won’t take up arms and the towns need to keep some protection so you are unlikely to be taking thousands). So have the dragon have minions and creatures that will fight for it. Have those forces come out to meet the army but the benefit to the players is that none of these forces are between them and the dragon now. The dragon won’t be coming out of its lair and the army can’t easily get up to its lair (and if they do will be a big mass of troops that can be knocked off the cliff). Don’t tell the players this, explain the route, maybe have NPCs explain the risks of the players insist then have there forces hit by dragon breath on the narrow paths, or the dragon hurl boulders down,
The army could be a great distraction allowing the players to sneak on the dragon or a hindrance depending how they use it, but it never need actually engage because, a 20 HP soldier with low AC and no shelling potion will die very quickly.
This is not the best advice, that moment with avantika was not the crux battle, in fact Matt brought her back later so the party never felt robbed of that fight. Matt did indeed roll dice behind the screen, you might not have seen it but he did it. But he also weighed up the situation. This is very different to that and should not just be described to the players as spectators, they should be in the middle of the battle fighting with the army, watching soldiers who are far weaker then them die, a more accurate description using critical roll would be the attack of the chroma conclave on Emon, Matt described the deaths of loads of people, including soldiers, while also having the players in the middle,
https://youtu.be/9EToAf4nhDw
For some ideas on ways you could run this forward this episode to 3:30 and watch till the end. It is a great example of how you can create that sense of a large battle without rolling hundreds of dice.
https://youtu.be/9QRg2YEkVLU
And in that one Matt actually has the party fight a black dragon with a small army, watch from 2:35ish. To get to the action (trap was set, many people hidden, lots of NPCs).
This is trying to stop the players out of game, personally I dislike doing this because you openly remove player agency, yea as DMs we secretly remove agency all the time, we do things based on what the players have done, tweak environments and encounters or mix things up just cos we can (personally never in combat but sometimes right before I will make tweaks). But telling your players flat you, you can’t do that, it isn’t logical, is bad. You can however have your NPCs make some valid points like, what if the dragon swoops from the air, or, where is its lair, how will an army climb up that narrow 1 person wide mountain pass, or, won’t the dragon see us marching on it. You can have a a grizzled veteran talk about a regiment that got burnt to a crisp before ether could let off an arrow, or have a soldier ask the hero’s if they can teach him how to cast a spell, or use that funky fighter ability so he might survive.
Have the characters realise and think about things and, if they insist anyway, then don’t have the dragon play fair, boulders dropped from on high, breath attacks as it swoops over the army and away, it’s minions attacking the army. Find ways in game ti encourage the characters to realise that the best approach is a sneaky approach, let them use the army as a distraction, maybe they tell them to scatter and run if they are the dragon and make it look like the dragon scared them away meanwhile party get to the lair un impeded and that is the benifit of the army, no pesky encounters using up resources on the way up to its lair. But never ever tell players, because you thought of X I am now going to make things harder, I have seen that approach actually drive players away from a DMs table because they thought what’s the point.
While you do recruit an army in Dragon Age: Origins, the point is not to use it to fight the dragon (end boss). The army is recruited to fight the army that the dragon has so that the heroes can reach the dragon. And the reason that you're able to recruit that army in the game is because the main character belongs to an organization with the authority to do so under the circumstances that occur in the game. From what I can tell, the PCs in this scenario lack that kind of leverage. The NPCs might be grateful for what the PCs did for them, but that doesn't compel them to become cannon fodder for the PCs. Have them offer up some alternative form of aid.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
I’m not suggesting the players are spectators and nothing more, I’m simply trying to explain how to save a lot of time and likely stress adding the army to what will already be a long and complex combat to run. Further down I do give some advice on how to handle the players attacking simultaneously with their forces. On the other hand you are right, and I did definitely overlook some points. Thanks for the feedback.
Be Excellent to one another. Rock on dude.
If you hate video games that much, why are you running your dragon like a video game?
Look, I get it. I don't play current video games either. They're predictable and boring. Better graphics has always been easier than better AI, and so you end up with beautifully rendered monsters who can't think beyond the end of their claws, and can't adapt tactics from their programmed script.
It doesn't mean the games are worthless. They have their strengths and weaknesses, just like anything else. If you want an immersive audio/visual experience, play a video game, I can't begin to compete; but if you want to wargame out a battle with a devious dragon, come to my tabletop. Bring your armies, too. Because that dragon does not live in a block of my memory waiting to be accessed whenever there is a relevant call for the data. It actively schemes and works against its enemies, spying, probing, and adapting to circumstances. Also because I don't need to have a script set out in advance for everything the players might or might not do, but instead I get to see what they're doing and focus my creative energy on responding to what they're doing with all the resources of a supernatural apex predator.
As others have illustrated here, low level minions are not threats to a dragon. Also, look at DMG p.92-4 and be real with your players about what they're asking for, which is essentially a suicide mission with expected casualty rates over 90%. Do the circumstances of their alliances really allow them to ask for such a sacrifice? And speaking of conscripting sacrificial minions, don't be shy about giving this dragon an army of followers too.
If you do go the route of large scale battle, yes see the mob attack rules (DMG p. 250) and handle things narratively that don't involve the players' rolls directly as others have suggested (especially if the dragon has an army too), but also look around to other systems to draw inspiration for running large scale battles. I personally run D&D games out of nostalgia at this point (ok, also WotC paroled themselves from the dungeon of my derision with 5e) but I have come to prefer another system, especially for combat focused games. I won't advocate for them by name here, but they use a system for large scale battles where each side's strength is represented by tokens, and then opposing commanders make opposed rolls which may result in the loss of tokens, and players who are not actively involved in a command role may make individual rolls to aid their side. Back to the WotC line, I believe there were rules in one of the 3e Combat & Tactics or Heros of Battle expansions that handled this too. But all of that assumes that you do in fact want to battle army vs. army, which again there is no reason to do if you don't want to do it.
Give your dragon some respect. Maybe extra HP or abilities if you want to (remember: expecting a dragon or any other monster to be exactly as represented in the MM is like expecting a human adventurer to have all 11s and 10s for ability scores), but give credit to an adult dragon's full range of abilities and resources. Your dragon wasn't programmed into existence to wait timelessly atop its hoard until the players and their armies walk into its lair. And the only way the PCs wishlist is lethal to it is if you let it sit there like a giant rock waiting for someone with a big enough lever to come along.
In short: this is not a time for "you can't do that"; this is a time for "are you sure you want to do that".
If the PC's want to pick a particular fight the challenge is to figure out what the PCs will do in that fight so that it doesn't turn into a simple narration.
I would push back against the suggestions that a dragon would wipe out the army as something to actually do to the PC's/ their army. They've been campaigning for a while. They've racked up accomplishments. Unless you're running a game with a Game of Thrones theme where seasons of exposition are wiped out in a few minutes of CGI and the entire tone shifts on a dime, you kind of have to let them have their army. So the question becomes:
How do you use that army?
A block of yeoman farmers with bows will eventually shoot down a dragon per DND mechanics. At the same time, the dragon will probably wipe ALL of them out with a single breath weapon in the middle of a move while flying. Thus part of the challenge. At the same time a smart dragon is going to use it's lair which is NOT likely to be army friendly and it's not going to fall into any obvious traps where it chases the PC"s out of its lair and into the army itself.
So one angle to take is to go back to your table and say "okay guys, here are some things that the officers in the army are bringing to you as concerns". If you're running an RP heavy game, you can literally create NPC's to represent the elected leaders of these groups (militias often voted for their officers; regular army is paid and thus had paid officers). They can ask the PC"s "what's the plan?" and raise all the questions this thread has so far. What's the plan for the lair? What's our plan to counter it's breath weapon? What's the plan if the dragon raises it's own army?
From a tactical point of view, you can also create layers of the battle(s). Sure the army can drop the dragon in an open fight but first someone has to deal with that necromancer it hired to raise an army to cover it. First someone has to clear the road the army's going to travel for ambushes. When the battle starts some people are going to need keep command (and maybe here you do something like give each PC a command and shift more into a tactical RP with "leadership skill checks" to give them dice to throw to help follow the narrative.
For my money, I would work to find ways to let the army be the thing that beats the dragon because that's the thing the PC's worked for and in many ways "earned".
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On a whim I put in a fight into a combat simulator using Goblins as my stand ins for Yoemen fighters and put 80 of them plus 20 goblin bosses (those volunteers who are actually competant warriors, retired adventurers, etc). Up against an adult red dragon here's what it kicked out:
I ran the same thing with an ancient red dragon and got:
So if it's just an adult red dragon I think the idea of an army of "cannon fodder" might be unfair. However if it's an ancient, you need to have a plan.
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I suggest for the army making it have a total set of hit points that would be visible to the party. 1 hp represents 1 humanoid.
This allows the players to have a sense of how much of their army is left during the fight. Then give the dragon an increased breath attack for strayfing and boost ac up to 21 as it will likely be in flight the whole time. If they bring it out of the sky bring it down to 19 ac as usual.
Should the dragon cripple the army (cripple is to bring to 1/4 total health) then have the army flee. By then the party should be able to finish the dragon themselves
This kind of suprises me I would have thought the dragon would have won more often, was the dragonflying high using hit and run tactics?
And also wiping out large numbers of the army with each breath? The first breath on round 1 (step 27) only hit 5 enemy - the shape of the breath was really weird, so not an accurate simulation at all.
The simulator has the dragon in melee with the enemy in round 3 (step 216) - so it is just doing multiattack in melee, even though it regained its breath weapon at the start of the round; and on the next goblin's turn it does Wing Attack which is only treated as hitting 1 enemy, rather than the 15 which are standing next to it; and it takes Tail attack as LA after the next goblin attack, and nothing thereafter - rather than another wing attack.
I don't think that the simulator works for dragons, or flying creatures.
Is the dragon's frightening presence being accounted for? Because that ought to be hitting everyone in the army and debuffing 90% or so of them.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
The general thing that determines army vs BBEG in 5e is number and type of ranged attacker; melee combatants will have limited ability to engage and are easily wiped out by area attacks, but it's quite hard to deal with a bunch of scouts.
I ran the fight kinda funny how the odds change if the dragon is a mother with 3 younger children….
In that particular scenario, Frightful Presence got used on the second turn- and then most enemy had disadvantage to hit.
Yeah it definitely wasn't going for an optional engagement and did rely a lot on "Claw/claw/bite" or "breath weapon" as it's primary choices. Even with disadvantage, a DC19 means that 10% of a force is going to resist and then 10% of the remaining feared force is going to recover every turn. Law of averages says that archers will eventually whittle the dragon down.
Now that said, you can obviously play the dragon smarter but I still think there's room here for the PC's to get what they want (we raised an army to take on this dragon) and still have the engagement be exciting, dynamic and fun.
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One thing I'd say should happen is have the Morale mechanics in play. Rather than rolling for every individual creature, I recommend either breaking the army into squads and roll a save per squad or to have a single roll for the entire army to see if morale breaks and it routs. Each round make the save, with increasing penalties as casualties accumulate. Squads/individuals that are affected by the frightening presence would obviously have disadvantage, and add a cumulative +1 to the DC per squad that flees and a cumulative +2 per squad that's killed.
And make sure you tell the players beforehand that these rules will be in play.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Really disagree with this, players should never be told outright no for something like this, that makes for an awful game where they feel they have no control or say. This is a very valid, very reasonable approach and tactic to take and just batting players wishes out of hand will make them less likely to continue playing.
Instead you should find ways to let them try, give the characters enough information to understand all the many risks involved in this, and if they insist on continuing down this route accept that you may well be killing off tons of NPC's as the dragon whittles away at the army refusing to engage them openly, uses the narrow and hard to get to routes up to it's lair to set traps, and drop boulders on the spread out single file troops etc.
But openly telling players, nope not letting you do that is awful DMing