When your players are in a long drawn out combat that they should be able to win, how do you handle the monotony? After a couple of rounds, the melee has the target flanked, the wizard and the rogue are off to the sides doing their thing. I want combat to be interesting and dynamic, how do you accomplish this? should the monsters be swapping targets, even if it means eating opportunity attacks?
By being descriptive with combat and using critical hits and misses for effect. For example, my Goliath Barbarian scored a crit on a goblin and did 50 something points of damage. So I said that he cleaved the Goblins head clear off his shoulders and the blade carried through allowing him to make an additional attack on the second goblin (at disadvantage, because he wasn't really trying to hit that one). Cause you know, why the hell not. It's not going to break the game. The same with critical fails. There are plenty of fail tables out there that can make things more dynamic. Someone slips, gets disarmed, blade gets wedged etc. It all adds to the combat, and the longer the combat goes the more chance there is that this type of thing happens.
Don't let yourself get bogged down in the repetition. Stay on top of your descriptions and narrative. It's easy to start strong and then struggle to maintain it. Write yourself a note or something, whatever it takes to keep you on your narrative game. And I'm not talking about waxing poetic for 5 mins on every swing and miss. But You have to keep it more than just "You miss. You hit, roll damage. He's dead." That's going to get boring real quick. Keep it moving. Move on to the next PC/monster straight away.
This is essentially a game design question. You should look at it that way, and read up on how game developers make combat in their own games fun.
You want to build encounters that allow players to make meaningful choices. You can't just throw some random monster at the players and expect that to be exciting. If they can just do the same thing they did in the last 3 encounters and win, asking them to do it a 4th time isn't going to be very interesting.
At the very least, you want to be using 2 or 3 different types of monsters that have easily observable mechanical differences. For example, wolves run fast, gang up on 1 enemy and can knock them prone. Goblins are weak but have ranged attacks and can hide or disengage easily. Hobgoblins have high AC, both melee and ranged attacks, and deal high damage if they have an ally within 5 feet of their target. These differences are easily observed by the player and force them to treat each enemy in a different way. An encounter with 1 hobgoblin, 2 goblin archers and 4 wolves is much more interesting than an encounter with 7 wolves or 7 goblins because the players have multiple valid strategies to choose from and none of them are clearly the one best option.
You can also vary:
The battlefield between encounters. There can be difficult terrain, different forms of cover and obscurement, climbable surfaces, pits, gaps wide enough that only some characters can easily jump across, traps, narrow walkways that require acrobatics checks to cross, and destructible elements like chandeliers that can be dropped or flammable objects.
The win conditions. Every once in a while, you can have an encounter where the goal isn't to just kill everyone in the opposite side. Maybe they have to destroy an object, stop a spellcaster's ritual, kill the enemy boss, rescue a hostage, or escape the dungeon alive.
Whether all enemies are accounted for from the start. Sometimes enemies call for reinforcements. Sometimes the PCs get an unexpected helping hand.
And like MajorPuddles mentioned, make an effort to not stop roleplaying when you're going through the motions of combat. A fight is never just a fight; the enemies have goals and needs and often personalities that should shine through in an encounter. Even if you have 4 goblins with the same stat block, they don't all have to be the same. Maybe 2 of them are competing to be promoted within their clan. Maybe 1 of them secretly wants to get one of the others killed. And going back to the theme of "mix different enemy types," those same goblins probably won't act the same way when they're on their own vs when they're being ordered to fight by a hobgoblin captain.
At my table we run tactical heavy encounters with each player only having 1 minute to take their turn. They're allowed to talk extensive tactics outside of combat. During combat they just do their own PC's actions with as little tactical debate in between. Otherwise each turn would take ages when they try to come to a decision on what the best course of action would be. When there is a lot going on the encounter can sometimes take way way longer then it should. Because its interesting it never feels like it. Even when some, more extreme, encounters take 90-120mins to resolve. Especially when they are about to resolve one situation and then "activate" the other rooms for example.
1. Add elevation. 3d encounter arena's are much more interesting and diverse. Can be something as simple as having trees where PC's can climb in, to areas with different plateau's. I love using the occasional Meazle. They're stealthy, jump on a stragglers back trying to choke them... Then teleport 120ft to any shadow it knows about. Doesn't have to be in Line of Sight. Meaning you can teleport the Meazle + PC it's holding to a higher point. Where the Meazle then tries to shove the PC down for falling damage.
2. Variable elements. Add additional, changing, elements to the battle arena. A lever on the higher plateau triggers spikes or moving traps on the level where the PC's are. Look at the PC game Black Guards for some interesting turn based elements. Something the players then have to keep in mind in regards to their positioning, which impacts other elements of combat.
3. Variable objectives. Have a deadly gas, water or something else flood the battle arena. Making it a time sensitive encounter. Will the PC's go through the effort of killing everything. Or KO the creature they came for. Taking it with them while trying to make a timely escape. Or perhaps they just need to protect a circle of clerics performing a ritual to fend off against fiends or the town will get over run. Or add something that would require a skill challenge, with perhaps team effort, to resolve in the midst of battle. Meaning they have to decide to spend an Action to either attack or perform a skill attempt.
4. Always use conditions. Orcs can just stand there and exchange blows. You can also have one orc grapple or knock a PC down, so that the other orcs can gang up for easy hits. Instead of just having goblins attacking a caravan. Add a goblin boss on a worg. The worg attacks a PC and gets a chance to knock them on the ground. Gaining advantage on consecutive attacks. Having melee goblins move in for adv. attacks and with their Nimble Escape run to safety again before the PC gets up. Hobgoblins can attempt disarm to remove shield/weapons from PC's. Use Shadows and Rot Grubs as well showing there are more dangers in the world. Have a creature on the upper plateau that can use Detect Thoughts. It'll know when a PC is nearby, even when the PC is a floor lower. Then attack the party with telepathic abilities, maybe even wipe the memories of spellcasters so they forget what spells they prepared for the day. Players will freak out when they don't even know what, nor where, the attacks are coming from.
5. I prefer to NOT use random encounters in the way most people use them. Rather have the encounter be meaningful and adding something to the overall story/situation. The Green Hag has 100's of Ogres working for her. So create 1 or 2 tailor made encounters that includes the Ogres and perhaps 1 or 2 with other beasties of the region. Drop them in where needed and otherwise just narrate how they hear big heavy thuds in the distance. Or how they see big lumbering shadows moving between the trees. Create the tension, foreshadowing, but doesn't necessarily have to end up into actual combat. When you do use a combat encounter like this have it be as dangerous as any other. Part of draining the PC's resources while they make their way to the destination.
6. Danger. Last saturday, during one of these "random encounters", I almost killed 2 PC's. That creates tension, sense of danger and makes them wonder what could be ahead. Lethality, chance of being dropped dead, often keeps players interested and invested as well. Can't remember last time I even made a Medium or lesser encounter. Those are just dull. Now you can use 6-8 encounters per adventuring day. As part of draining your PC's resources. I rather not use that many encounters and instead make each encounter more interesting/dangerous. Those drain resources just as well. Especially when the players misjudge and blow through many of their abilities early on and still have another 3 hard-deadly encounters to go. Of course this has to be adjusted per group. If your players aren't tactical, or the DM isn't in line with how tactically capable the players are, then it won't really work. Then you'd better play it safe with some easier encounters in the mix.
In this regard I personally love using Strongholds & Followers by Matt Colville. Enemies have lairs as well. Providing them with extra buffs. Orc stronghold could provide all the orcs in the vicinity a crit range of 19-20. Wonder how your players will be surprised when they suddenly appear to hit harder, more regularly. With the leading orc having Line of Sight in that combat it'll grant specific boons to his nearby underlings.
7. Actual combat slow down... Well if you want to use many minions. You can run them on the same initiative, or in the same manner, as a Lair Action. This will speed up the game mechanically, but does require you to narrate the Lair Action properly. Otherwise you won't convey the feeling of being surrounded by tons of minions. Think of a necromancer's laboratory where, just out of reach from the light source, the walls and ceiling seem to come alive. Hundreds if not Thousands of Crawling Claws spring to action. Crawling towards the PCs or falling from the ceiling on top of them. Dealing a set amount of damage and/or DC save vs a Status Condition. Perfect chance for a druid to drop her Moonbeam in a choke point to have those claws be destroyed like flies against an electric zapper.
If you run 3-4 monsters of the same type. And you run multiple of such groups in your combat. Try having them act on the same initiative if possible to speed things up. If you are running towards the end of combat. And the PC's have clearly won/achieved the objective... Then the clean up of remaining creatures can be a chore. You can then ask your players what they PC's would do and just narrate a swift conclusion. Or you can have enemies break their shields or something that drops their AC allowing PC's easier hits for expediency. This part of the game is pretty much the only moment I fudge die rolls to speed things up, since the outcome is clear anyhow. During this stage you can also, quite easily, explain how the enemies lost their Moral. They'll behave more erratic and panicked. They will make mistakes indeed leading to possible OA's against them as they try to flee or do something as a last resort. Maybe some self sacrifice/kamikazi action.
8. Narrate the action. Have your players describe their spells and melee swings. Then you describe how the creatures react. This creates more of a natural flow. Be careful not to go overboard or it'll slow combat down quite a bit. A simple sentence in regards to exchanging blows, blocking with the shield leading to sparks and loud clangs echoing through the grotto. Or the Ogre charges at you, saliva dropping from his large tusks. It looms over you as its large club comes crashing down. You manage to duck as the club slams into the tree next to you. The heavy impact bending the thin tree midway through. Or the club comes crashing down. Crushing the falling tree stump behind you. Wood splinters fly around. You shot the bolt at this ogre. His fierce gaze focusses on you as it charges towards you. A loud hollar leaves it throat with spittle dripping down its chin. Schrubs and tree branches get destroyed as it makes it way through the treeline. Greataxe raised and swings at your head. Basically throw in some descriptions that are bit more lengthy dramatic combined with short 1 sentence narration. Don't forget to mention the feel of the environment or the smell of copper/blood hanging in the air to spruce things up a bit. Takes work, so practice and also delve into some video's and such about the topics/environments you want to describe. Not just in combat, but also during exploration.
9. Don't over use tables. There are tables for everything. Rolling on everything slows the pace down drastically as well. Keep things to a minimum. You don't need a critical fail table. Just narrate something comedic. Like the ogre rushing towards you. It stumbles over a thick tree log and his swing goes wide. If you narrate that the ogre falls on his knees (Prone) or something then that is what happens. No need for a table. Same applies to critical. Only table I use is a simplistic Injury table similar to what Mark Hulmes uses. And various tables in regards to exploration daily events that occur, gathering herbs and such while exploring and stuff like that.
10. Movement skills. This is one of the few things I liked from 4e. There were items and skills that let you impact the battle arena by movement. Either you allows a team member to get an extra move, or you forced the enemy to move in a certain way that was beneficial to you. In short. Look over monster stat blocks from 4e and copy some elements. Hobgoblin leaders could get a martial ability. As a bonus action they command nearby allies to move an additional 5-10ft. This means those allies can gain a more advantageous position on the PCs. The same if you add pull or push abilities as part of an attack they can use. On top of the attacks in 5e that provide a save check against condition status. Try to create your own opponents as well like a warrior that can use entangle in a 5ft area around him as a bonus action. Or a skeleton that has a necrotic aura dealing x damage to everyone adjacent to it. In short try to give creatures interesting additional skills/abilities they can use as a bonus action. Most don't have that and will also aid in creating a better Action Economy for the monster vs player as well. Can also do it in a way that you create interesting items that provide the monster such abilities. Items that your players can then pick up and use. When Players see monsters doing interesting things they might be more invested to see if it is a cool item which would enable their future exploits.
I like to take InquisitiveCoder's idea in a slightly different direction; along with different personalities and/or goals for each creature in a fight, also give a major change in tactics/strategy when the fight reaches certain points.
I had a fight just recently where the group was outnumbered 3:1 however most of the creatures weren't a major threat on their own. Each creature had something special about them that made them useful to the fight, but only if it was used in concert with the other creatures on the board. The Scarecrow's gaze used to potentially paralyze a PC, the Orcs used to close in on the players when they started to become more mobile, the Skeletal Abomination (homebrew) had the stats of the Orc Warchief for extra damage and empowering the other Orcs. Each of these were also Undead, so that made it even more fun. Their abilities making them pretty nasty to fight, but working together, it made them a bigger threat.
When the smaller creatures were mostly destroyed, leaving a few undead Orcs and the Skeletal Abomination on the field, I needed to make the fight feel less like mowing down some mooks. The Skeletal Abomination changed form, getting larger and changing shape to appear as though a new head was forming in its chest. This head could protrude from the chest or back and would exhale a blast of necrotic energy (chill touch) as well as the multi-attack the Warchief statblock already had. The damage from combat up to this point was still apparent, however it seemed as though this creature wasn't tiring from it. I had given it some temporary HP when it changed, just enough to give it a turn or two to show off that it had changed tactics.
That's a very simple change to make a creature interesting during a combat, mechanically. You can also use that kind of approach with their personality too. The most recent one I've used was a fight with a spell casting Gnome and his pet Beholder. During the first half of the battle that Gnome acted as though he had everything under control. He would use support attacks and spells to help the Beholder while the players fought. He stayed out of combat as much as he could while calling out taunts and jeers. Then, as is expected, the players downed the Beholder, that Gnome ran like the wind. He retreated into his sanctum, locked the door and became the equivalent of a 3 year old child not wanting to leave the toy aisle. The players were ready for some epic showdown, but instead got stuck dragging a flailing gnome, throwing the ultimate temper-tantrum, into the Seelie Court for his judgement.
When your players are in a long drawn out combat that they should be able to win, how do you handle the monotony? After a couple of rounds, the melee has the target flanked, the wizard and the rogue are off to the sides doing their thing. I want combat to be interesting and dynamic, how do you accomplish this? should the monsters be swapping targets, even if it means eating opportunity attacks?
By being descriptive with combat and using critical hits and misses for effect. For example, my Goliath Barbarian scored a crit on a goblin and did 50 something points of damage. So I said that he cleaved the Goblins head clear off his shoulders and the blade carried through allowing him to make an additional attack on the second goblin (at disadvantage, because he wasn't really trying to hit that one). Cause you know, why the hell not. It's not going to break the game. The same with critical fails. There are plenty of fail tables out there that can make things more dynamic. Someone slips, gets disarmed, blade gets wedged etc. It all adds to the combat, and the longer the combat goes the more chance there is that this type of thing happens.
Don't let yourself get bogged down in the repetition. Stay on top of your descriptions and narrative. It's easy to start strong and then struggle to maintain it. Write yourself a note or something, whatever it takes to keep you on your narrative game. And I'm not talking about waxing poetic for 5 mins on every swing and miss. But You have to keep it more than just "You miss. You hit, roll damage. He's dead." That's going to get boring real quick. Keep it moving. Move on to the next PC/monster straight away.
This is essentially a game design question. You should look at it that way, and read up on how game developers make combat in their own games fun.
You want to build encounters that allow players to make meaningful choices. You can't just throw some random monster at the players and expect that to be exciting. If they can just do the same thing they did in the last 3 encounters and win, asking them to do it a 4th time isn't going to be very interesting.
At the very least, you want to be using 2 or 3 different types of monsters that have easily observable mechanical differences. For example, wolves run fast, gang up on 1 enemy and can knock them prone. Goblins are weak but have ranged attacks and can hide or disengage easily. Hobgoblins have high AC, both melee and ranged attacks, and deal high damage if they have an ally within 5 feet of their target. These differences are easily observed by the player and force them to treat each enemy in a different way. An encounter with 1 hobgoblin, 2 goblin archers and 4 wolves is much more interesting than an encounter with 7 wolves or 7 goblins because the players have multiple valid strategies to choose from and none of them are clearly the one best option.
You can also vary:
And like MajorPuddles mentioned, make an effort to not stop roleplaying when you're going through the motions of combat. A fight is never just a fight; the enemies have goals and needs and often personalities that should shine through in an encounter. Even if you have 4 goblins with the same stat block, they don't all have to be the same. Maybe 2 of them are competing to be promoted within their clan. Maybe 1 of them secretly wants to get one of the others killed. And going back to the theme of "mix different enemy types," those same goblins probably won't act the same way when they're on their own vs when they're being ordered to fight by a hobgoblin captain.
The Forum Infestation (TM)
I try to run combat pretty fast to begin with, but if this comes up I usually:
At my table we run tactical heavy encounters with each player only having 1 minute to take their turn. They're allowed to talk extensive tactics outside of combat. During combat they just do their own PC's actions with as little tactical debate in between. Otherwise each turn would take ages when they try to come to a decision on what the best course of action would be. When there is a lot going on the encounter can sometimes take way way longer then it should. Because its interesting it never feels like it. Even when some, more extreme, encounters take 90-120mins to resolve. Especially when they are about to resolve one situation and then "activate" the other rooms for example.
1. Add elevation. 3d encounter arena's are much more interesting and diverse. Can be something as simple as having trees where PC's can climb in, to areas with different plateau's. I love using the occasional Meazle. They're stealthy, jump on a stragglers back trying to choke them... Then teleport 120ft to any shadow it knows about. Doesn't have to be in Line of Sight. Meaning you can teleport the Meazle + PC it's holding to a higher point. Where the Meazle then tries to shove the PC down for falling damage.
2. Variable elements. Add additional, changing, elements to the battle arena. A lever on the higher plateau triggers spikes or moving traps on the level where the PC's are. Look at the PC game Black Guards for some interesting turn based elements. Something the players then have to keep in mind in regards to their positioning, which impacts other elements of combat.
3. Variable objectives. Have a deadly gas, water or something else flood the battle arena. Making it a time sensitive encounter. Will the PC's go through the effort of killing everything. Or KO the creature they came for. Taking it with them while trying to make a timely escape. Or perhaps they just need to protect a circle of clerics performing a ritual to fend off against fiends or the town will get over run. Or add something that would require a skill challenge, with perhaps team effort, to resolve in the midst of battle. Meaning they have to decide to spend an Action to either attack or perform a skill attempt.
4. Always use conditions. Orcs can just stand there and exchange blows. You can also have one orc grapple or knock a PC down, so that the other orcs can gang up for easy hits. Instead of just having goblins attacking a caravan. Add a goblin boss on a worg. The worg attacks a PC and gets a chance to knock them on the ground. Gaining advantage on consecutive attacks. Having melee goblins move in for adv. attacks and with their Nimble Escape run to safety again before the PC gets up. Hobgoblins can attempt disarm to remove shield/weapons from PC's. Use Shadows and Rot Grubs as well showing there are more dangers in the world. Have a creature on the upper plateau that can use Detect Thoughts. It'll know when a PC is nearby, even when the PC is a floor lower. Then attack the party with telepathic abilities, maybe even wipe the memories of spellcasters so they forget what spells they prepared for the day. Players will freak out when they don't even know what, nor where, the attacks are coming from.
5. I prefer to NOT use random encounters in the way most people use them. Rather have the encounter be meaningful and adding something to the overall story/situation. The Green Hag has 100's of Ogres working for her. So create 1 or 2 tailor made encounters that includes the Ogres and perhaps 1 or 2 with other beasties of the region. Drop them in where needed and otherwise just narrate how they hear big heavy thuds in the distance. Or how they see big lumbering shadows moving between the trees. Create the tension, foreshadowing, but doesn't necessarily have to end up into actual combat. When you do use a combat encounter like this have it be as dangerous as any other. Part of draining the PC's resources while they make their way to the destination.
6. Danger. Last saturday, during one of these "random encounters", I almost killed 2 PC's. That creates tension, sense of danger and makes them wonder what could be ahead. Lethality, chance of being dropped dead, often keeps players interested and invested as well. Can't remember last time I even made a Medium or lesser encounter. Those are just dull. Now you can use 6-8 encounters per adventuring day. As part of draining your PC's resources. I rather not use that many encounters and instead make each encounter more interesting/dangerous. Those drain resources just as well. Especially when the players misjudge and blow through many of their abilities early on and still have another 3 hard-deadly encounters to go. Of course this has to be adjusted per group. If your players aren't tactical, or the DM isn't in line with how tactically capable the players are, then it won't really work. Then you'd better play it safe with some easier encounters in the mix.
In this regard I personally love using Strongholds & Followers by Matt Colville. Enemies have lairs as well. Providing them with extra buffs. Orc stronghold could provide all the orcs in the vicinity a crit range of 19-20. Wonder how your players will be surprised when they suddenly appear to hit harder, more regularly. With the leading orc having Line of Sight in that combat it'll grant specific boons to his nearby underlings.
7. Actual combat slow down... Well if you want to use many minions. You can run them on the same initiative, or in the same manner, as a Lair Action. This will speed up the game mechanically, but does require you to narrate the Lair Action properly. Otherwise you won't convey the feeling of being surrounded by tons of minions. Think of a necromancer's laboratory where, just out of reach from the light source, the walls and ceiling seem to come alive. Hundreds if not Thousands of Crawling Claws spring to action. Crawling towards the PCs or falling from the ceiling on top of them. Dealing a set amount of damage and/or DC save vs a Status Condition. Perfect chance for a druid to drop her Moonbeam in a choke point to have those claws be destroyed like flies against an electric zapper.
If you run 3-4 monsters of the same type. And you run multiple of such groups in your combat. Try having them act on the same initiative if possible to speed things up.
If you are running towards the end of combat. And the PC's have clearly won/achieved the objective... Then the clean up of remaining creatures can be a chore. You can then ask your players what they PC's would do and just narrate a swift conclusion. Or you can have enemies break their shields or something that drops their AC allowing PC's easier hits for expediency. This part of the game is pretty much the only moment I fudge die rolls to speed things up, since the outcome is clear anyhow. During this stage you can also, quite easily, explain how the enemies lost their Moral. They'll behave more erratic and panicked. They will make mistakes indeed leading to possible OA's against them as they try to flee or do something as a last resort. Maybe some self sacrifice/kamikazi action.
8. Narrate the action. Have your players describe their spells and melee swings. Then you describe how the creatures react. This creates more of a natural flow. Be careful not to go overboard or it'll slow combat down quite a bit. A simple sentence in regards to exchanging blows, blocking with the shield leading to sparks and loud clangs echoing through the grotto. Or the Ogre charges at you, saliva dropping from his large tusks. It looms over you as its large club comes crashing down. You manage to duck as the club slams into the tree next to you. The heavy impact bending the thin tree midway through. Or the club comes crashing down. Crushing the falling tree stump behind you. Wood splinters fly around. You shot the bolt at this ogre. His fierce gaze focusses on you as it charges towards you. A loud hollar leaves it throat with spittle dripping down its chin. Schrubs and tree branches get destroyed as it makes it way through the treeline. Greataxe raised and swings at your head. Basically throw in some descriptions that are bit more lengthy dramatic combined with short 1 sentence narration. Don't forget to mention the feel of the environment or the smell of copper/blood hanging in the air to spruce things up a bit. Takes work, so practice and also delve into some video's and such about the topics/environments you want to describe. Not just in combat, but also during exploration.
9. Don't over use tables. There are tables for everything. Rolling on everything slows the pace down drastically as well. Keep things to a minimum. You don't need a critical fail table. Just narrate something comedic. Like the ogre rushing towards you. It stumbles over a thick tree log and his swing goes wide. If you narrate that the ogre falls on his knees (Prone) or something then that is what happens. No need for a table. Same applies to critical. Only table I use is a simplistic Injury table similar to what Mark Hulmes uses. And various tables in regards to exploration daily events that occur, gathering herbs and such while exploring and stuff like that.
10. Movement skills. This is one of the few things I liked from 4e. There were items and skills that let you impact the battle arena by movement. Either you allows a team member to get an extra move, or you forced the enemy to move in a certain way that was beneficial to you. In short. Look over monster stat blocks from 4e and copy some elements. Hobgoblin leaders could get a martial ability. As a bonus action they command nearby allies to move an additional 5-10ft. This means those allies can gain a more advantageous position on the PCs. The same if you add pull or push abilities as part of an attack they can use. On top of the attacks in 5e that provide a save check against condition status. Try to create your own opponents as well like a warrior that can use entangle in a 5ft area around him as a bonus action. Or a skeleton that has a necrotic aura dealing x damage to everyone adjacent to it. In short try to give creatures interesting additional skills/abilities they can use as a bonus action. Most don't have that and will also aid in creating a better Action Economy for the monster vs player as well. Can also do it in a way that you create interesting items that provide the monster such abilities. Items that your players can then pick up and use. When Players see monsters doing interesting things they might be more invested to see if it is a cool item which would enable their future exploits.
I like to take InquisitiveCoder's idea in a slightly different direction; along with different personalities and/or goals for each creature in a fight, also give a major change in tactics/strategy when the fight reaches certain points.
I had a fight just recently where the group was outnumbered 3:1 however most of the creatures weren't a major threat on their own. Each creature had something special about them that made them useful to the fight, but only if it was used in concert with the other creatures on the board. The Scarecrow's gaze used to potentially paralyze a PC, the Orcs used to close in on the players when they started to become more mobile, the Skeletal Abomination (homebrew) had the stats of the Orc Warchief for extra damage and empowering the other Orcs. Each of these were also Undead, so that made it even more fun. Their abilities making them pretty nasty to fight, but working together, it made them a bigger threat.
When the smaller creatures were mostly destroyed, leaving a few undead Orcs and the Skeletal Abomination on the field, I needed to make the fight feel less like mowing down some mooks. The Skeletal Abomination changed form, getting larger and changing shape to appear as though a new head was forming in its chest. This head could protrude from the chest or back and would exhale a blast of necrotic energy (chill touch) as well as the multi-attack the Warchief statblock already had. The damage from combat up to this point was still apparent, however it seemed as though this creature wasn't tiring from it. I had given it some temporary HP when it changed, just enough to give it a turn or two to show off that it had changed tactics.
That's a very simple change to make a creature interesting during a combat, mechanically. You can also use that kind of approach with their personality too. The most recent one I've used was a fight with a spell casting Gnome and his pet Beholder. During the first half of the battle that Gnome acted as though he had everything under control. He would use support attacks and spells to help the Beholder while the players fought. He stayed out of combat as much as he could while calling out taunts and jeers. Then, as is expected, the players downed the Beholder, that Gnome ran like the wind. He retreated into his sanctum, locked the door and became the equivalent of a 3 year old child not wanting to leave the toy aisle. The players were ready for some epic showdown, but instead got stuck dragging a flailing gnome, throwing the ultimate temper-tantrum, into the Seelie Court for his judgement.