Hi all Fairly New DM here, 

So, for my homebrew campaign my party have just reach a time in the campaign where I want to do a big reveal to two out of the three players, about why my warlock player made their pact to become a warlock. The story is fair simply that the previous adventuring party they were part of, was wiped out on a mission gone horribly wrong. As the only survivor they made the pact, so they had the power to stop something like that ever happening again. 

Any way to reveal this: I want to hook the party up to basically the fantasy version of VR, where the whole party has to relive the warlocks' memories of the battle that killed their friends and left them for dead. So, I basically want to kill my party! I have had them all create new characters that they don't know why they need, and the warlock has created a version of herself as a different class pre warlock pact. I know the environment and how I want the battle to take place. But how much more powerful do I make the enemies in order to kill the party but still make it a fun fight that doesn't wipe them out in round one or leave them feeling discouraged and hopeless (even though it will be hopeless in the end I want to drag it out as much as possible!)? Eg how many levels stronger should they be, should I just outnumber them or attack in waves of enemies or something else.

Hope this make sense and you guys have some suggestions for me. Thanks Delphi

The last time I did this (which was... in a prior edition), I ran the PCs through a gauntlet -- each individual encounter was nasty but beatable, and somewhere around the third encounter they ran out of steam.

Short of subtle casting power word kill on a character of less than 100hp there never can be a guarantee what will happenith a single or even single group of enemies. Depending what level the party is a spell like polymorph could give the party plenty of time to escape.

How did you character escape or did their patron raise them from the dead?

One option , as long as the party can not teleport away is a constant never ending stream on enemies. The more they kill the more that take their place until eventually the party is defeated.

Well, since the entire concept is a railroad, you can just really lean into it. Have them do the fights and just fudge your rolls to get whatever result you desire.

And I'm not meaning that as an insult, so I hope it doesn't come across as such. I'm not trying to judge. But basically, you know before the start how things must end. So, just do what you need to make it end that way. Otherwise, no matter what you come up with, monster-wise, there's always a chance the dice will not behave, and the encounters will be too easy, or result in a tpk before you want it to happen. Also, are you certain the other players are that interested in this warlock's backstory that they want to put down and ignore their own characters (even temporarily) and play through someone else's story? They may find it really fun, that will depend on the group, but it can be a big swing. Not to mention the warlock. Are you sure this is how the player want their character's story revealed, rather than doing it on their own terms through their own dialogue? 

Another alternative is to narrate it without playing it through. Which is sometimes a good choice but can also get boring really quickly if its more than a paragraph or two in length. But there, again, we run into you telling the character's story.

I think waves of enemies may be the way to go. And the warlock will be left for dead not fully killed but you make a good point I should probably plan something in case I'm too successful 

No you've not offended me in any way  I'm expecting people to give me advice with out being able to give full background on all my players characters and my players personalities and what they will enjoy because that's hard to describe. When starting our campaign I explained my concepts to them and they all created strong back stories each and agreed that they would not know each others histories yet being new to working together but that they would be revealed as part of the campaign and world building and as their bond as party strengthened. And all the players are intrigued by the need to create a new character for reasons they don't fully understand yet. If anything I'm worried they will get too attached to the new characters rather than them being unwilling to not play their main characters for the whole session!

My dm skills are stronger for story telling than combat. I've been side barring with the warlock so she knows a lot more than the other two players about the next session and to expect a flash back for lack of a better phrase and so we can drive more into the role playing aspects this session and cover some emotive revelations. It's been hard to balance preparing the player and getting get feedback on what she is comfortable with with out revealing everything.

I'm trying to write into my plan lots of prompts for myself to remind me to make sure I give all the players plenty of opportunities to express who the feel or react to the situation. 

I would have a SWARM. I put it in all caps because I don’t mean like 100-300 orcs… I mean 100,000s. They should be able to see the size of the army, feel the sweat dripping down their faces as they face utter destruction. Maybe give them a reason for standing such as-

they’re magic user friends are casting a ritual that will get them out of there, but in a twist of fate the party sacrifices themselves so the others can escape, or you could do essentially the same thing buts it’s a town/city being evacuated. And if they survive like 10+ rounds then throw something big in, like an ancient dragon or lich (probably the ones leading the armies).

I think I would work backwards from the question, "what information am I trying to impart to the players?" rather than just focusing on questions like "how many creatures" and "how tough are they". Is this just a lore dump for the warlock's backstory, or is what happened in the past going to become very relevant in the present at some point?

If you're doing a heroic last stand kind of scenario, maybe the enemy they fought used some tactic they would do well to find a counter for, or revealed themselves in some way if the BBEG's identity is a mystery

You could also add in a lethal puzzle or trap element to it, something the new party will eventually encounter -- and knowing what happened to the old party, and what they did to try to solve it, might be the key they need to get through it when it's their turn

Basically, if you want it to be more than a fun one-shot diversion from the main campaign, give the players a reason to pay attention to the details in this shared memory

Quote from s9dyn2kpyf >>

I would have a SWARM. I put it in all caps because I don’t mean like 100-300 orcs… I mean 100,000s. They should be able to see the size of the army, feel the sweat dripping down their faces as they face utter destruction. Maybe give them a reason for standing such as-

they’re magic user friends are casting a ritual that will get them out of there, but in a twist of fate the party sacrifices themselves so the others can escape, or you could do essentially the same thing buts it’s a town/city being evacuated. And if they survive like 10+ rounds then throw something big in, like an ancient dragon or lich (probably the ones leading the armies).

 This is more tangential than anything, but you get diminishing returns on hoards that big. Once things get to the crunch of melee, each player can fight about 8 at once (one occupying each adjacent space, and then maybe a few more in the case of reach weapons though I imagine hacking and slashing around your fellow soldiers with a poleaxe would be awkward), and with all those enemies attacking at once and enough to immediately fill the place of any fallen, the number stops mattering statistically. You could achieve the same effect by saying "an army" as you could "100,000 orcs." The players probably won't survive much past the first wave, so there's little difference to them between 200 or 200,000.

One thing I did to my players, was write up a stat block for an enemy that would take the party about 2 rounds to kill... and just start adding 1-2 more of those on each round of combat.  The first few rounds will be easy, but the enemies build up faster than the party can cut them down. 

In my campaign, the party was facing a demonic invasion via planar rift -- while the actual boss was pelting them with AoE attacks every round too.  It could just as easily be a "last line of defense" scenario where the party faces a legion of invaders that's breaking through their main defenses.  I have no idea whether that kind of a combat setup is even compatible with the story you have in mind, but it did have the effect you're going for (a hopeless battle that's not an instant TPK) and my players enjoyed it.

I think it's a horrible idea, and if I was a player, I'd never sit at your table again. 

Quote from Oakbrow >>

I think it's a horrible idea, and if I was a player, I'd never sit at your table again. 

 Well thank goodness your not a player lol. They’ve literally said that the players already agreed to doing this, so it doesn’t matter if you think they should/shouldn’t.

Quote from CharlesThePlant >>

Quote from s9dyn2kpyf >>

I would have a SWARM. I put it in all caps because I don’t mean like 100-300 orcs… I mean 100,000s. They should be able to see the size of the army, feel the sweat dripping down their faces as they face utter destruction. Maybe give them a reason for standing such as-

they’re magic user friends are casting a ritual that will get them out of there, but in a twist of fate the party sacrifices themselves so the others can escape, or you could do essentially the same thing buts it’s a town/city being evacuated. And if they survive like 10+ rounds then throw something big in, like an ancient dragon or lich (probably the ones leading the armies).

 This is more tangential than anything, but you get diminishing returns on hoards that big. Once things get to the crunch of melee, each player can fight about 8 at once (one occupying each adjacent space, and then maybe a few more in the case of reach weapons though I imagine hacking and slashing around your fellow soldiers with a poleaxe would be awkward), and with all those enemies attacking at once and enough to immediately fill the place of any fallen, the number stops mattering statistically. You could achieve the same effect by saying "an army" as you could "100,000 orcs." The players probably won't survive much past the first wave, so there's little difference to them between 200 or 200,000.

 The reason I’d want it to be said that the horde is that big- the feeling of impending doom. If you say “an army approachs” then they have no idea the magnitude of what they face, but if you say “an army numbering in the 100,000s marches, their footsteps echoing like thunder, only surpassed in volume by their horrid battle cries” then the players get an idea of how hopeless the fight is, so then you throw in a reason not to run, such as they’re buying someone/something time. Plus I like the horde death because- if you’re faced with an opponent that’s WAY stronger then you then they’ll just one-shot you and move on, but with a horde you’ll be able to better control how long the party lasts before they die.

I like that idea - they are going to be in a labyrinth like ravine in a wasteland with caves higher up and lots of crevices. So, I'm planning archers from above in the caves and extra waves of fighters (part of a shadow syndicate of thieves and other criminals) coming in gradually from different directions at ground level. So, adjusting numbers to flex with how easy or hard they are finding the fight until it feels right to end things sounds like a good plan.

 Even though the players have agreed to be invested in each others back story, and agree to play these one shots (please make it clear they are for a one shot so you don't have the case they get to attached to them), they might still be unhappy if they realize nothing they did the entire session mattered or had any outcome.

So think of some way their choices and actions/performance in this one shot can effect things in the main campaign. Maybe they'll be defending the main passage and they'll have to pick an NPC to guard a side one, and NPC to escape to take the McGuffin to safety.  Now their choice will effect which NPC they'll see in the main campaign. Or maybe they'll have a chance to scout and know of two possible path with two diff Lt. a ranger and a barbarian.  If the party confronts and kills one they'll have the other in the main campaign.

Anyway I'm sure you can think of a bunch of ways their choices can effect the main campaign. Inserting 2 or 3 could go a long way to making the night matter for everyone.

tl;dr Even if the conclusion is already known, you can provide a measure player agency. The more you can provide the more rewarding the one-shot will be.

Others have said this, but I’ll say it too: Your players need to have something they do actually matter. If your players wanted to hear a story with a fixed outcome, then they would have just watched a movie or read a book. But they chose to sit down at your D&D table – at least in part – because their decisions and actions affect the outcome of their shared story.

So they may all die in this flashback thing. But somehow, how well they did before death should matter, at least in a small way, to the main campaign and world.

Also, warn all your players (not just the warlock) that their situation may not go so well, so they’re at least somewhat prepared when almost all of them die. And if you’ve worried your players might get too attached to these characters, then also warn them that they’ll only be using these characters for approximately one session. Or you could even tell them up front: We’re running one session with these characters, and they’re all going to die.

As for running the combats, I’d advise you to have a couple tough encounters of monsters, and then more and more reinforcements. Another idea of preventing it from ending too early is to start with easy encounters that grow progressively harder. However, to make sure that whether it is due to terrain, the way the monsters are oriented, or something the characters to have defend, that they can’t just run away, escape, and survive.

And flashbacks are tricky. You can’t have the characters mess with the future too much. So the party needs to die, and the warlock somehow needs to survive. You say you want the warlock to be “left for dead” and you may have it already, but you need a plan for how that will happen mechanically – unless you change something, the only way the warlock won’t have to make death saves is if a monster intentionally knocked them unconscious. So maybe a monster knocks them out to take them prisoner and then dies? I dunno, how the monster would die seems pretty iffy. Or they could be lying unconscious when their Patron reaches out to them, makes a pact, and saves them? You’ll need to come up with something for this if you haven’t already.

So, there, long rant over. Hopefully at least one of the pieces of advice I gave here will prove to be useful.

In answer to your question, the way you do this is by not having a HP value for the enemies.  Full stop.  You make them literally indestructible.  Take it from someone who has been down this road a number of times: anything else you do, the players will find a way to circumvent it.  Some folks have suggested waves of enemies — that’s also a fine solution, as long as the waves don’t stop coming until everyone is unconscious.

But.

I recommend having the warlock player write and narrate this, rather than playing it out.  This is their backstory we are talking about, and it does not make sense for the other players to have agency regarding how it goes, particularly since it is over and done with.  Sure, it’ll probably be fine, but in a worst case scenario you could end up making something canon to the warlock’s backstory that they don’t want, and speaking personally, I wouldn’t consider that risk worthwhile.

Also, as other folks have noted, if you’re not giving the players agency, there’s no reason to ask for their input.  Or to put it another way, only ever ask for the players’ input if it is going to matter.