Combat makes up a decent chunk of most campaigns and the setup can make it either a blip that no one will remember to one of the most memorable parts of the campaign. I've been looking for various ways to make combat more fun in my campaign and was eager to see if people here have any ideas they'd like to share. I'm looking for anything that gives alternate win conditions, gives the players new ways to interact with the environment, forces special behaviour or anything to make a combat encounter unique.

I'll start with some of my favourites.

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Battle Complications

Anything that makes combat more complicated than "kill this list of people". Special conditions and anything in that direction.

-- Ritual in Progress Someone is casting a terrifyingly strong ritual and it must be stopped by either destroying conduits, removing sacrificial materials or killing the casters. Typically this is something that allows enemies that wouldn't be very dangerous otherwise to have a far greater effect simply because the party can't focus them down, needing to spend their time on the ritual being stopped. If they fail, the possible punishment should sound quite bad, so that they feel a strong urge to stop what is happening.

-- Ritual-borne Foe Similar to the last one, but this time it's not a time limit but rather something that requires a novel way to defeat. An enemy that is incredibly powerful and dangerous to the party being kept alive or present by as summoner or necromancer of some sort, or alternatively by a contraption. Players must destroy or deactivate the source to defeat the opponent before it defeats them. This may be through directed combat or through a puzzle of sorts in the middle of combat.

-- Constant Reinforcements The BBEG is constantly calling forth fiends from another realm or some device creates more and more small creatures swarming the players. These elements make sure that the players can't just clear out the room, leaving the big bad guy in the middle helplessly overwhelmed by sheer action-economy. Instead, they have to either win quickly or alternatively do their best to disable the sources of enemies.

Environmental Hazards

These are difficult to design but great when they work out. Hazards in this case is taken in a very general sense.

-- Unstable Ground The ground that both the players and enemies are standing on is unstable and may break away easily. May it be as the result of some level being pulled, an attack hitting the ground or maybe just random chance. As long as the players have some chance to react to what is happening, this can include anything that can make ground unsafe to simply stand on. This makes certain types of ground that are stable very favorable and possibly worth fighting over. Bonus if the enemies have ways to contest ground, just like the players can.

-- The Floor is Lava The ground erupts in Flame, flashes of radiant beams hit the combat arena or meteors strike the area. Very similar to unstable ground, but more temporary, are occasional AoEs hitting the ground. Once again, these are best when the players feel like they can really work around them sensibly.

-- Bottomless Pit (Maybe with a bottom) A pit that people can be thrown into. This urges people to finally realize that shove is a thing. Don't make it too easy to fall into though, falling to your death into such a pit with little chance to safe yourself won't feel great. Make sure that they get a chance to save themselves or be saved if they do fall!

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These are what comes to mind immediately. What interesting mechanics have you used to change your combat into something the party ended up finding really thrilling or memorable? I myself play DnD 5e, but I would love to hear your stories from other systems that I may implement in my campaign too!