This will be the first in what I expect will be a short series of posts dealing with "trick" encounters - monsters, treasures, and other things. The ones that have something up their sleeve that makes them a bit different than a normal monster encounter. Usually, because there is some clever unknown that preys on the expectations of the players and forces them into different decisions to deal with the monster.
There is an old trope of the movies - kill the boss, his fort falls. Why? Seems silly, and sometimes it clearly is the product of "it's in the script" but with no explanation. But it doesn't have to be silly. You can use this - with some different trappings - for a real treat in your own games.
Careful, That's A Load-Bearing Monster
The basic idea is, the health of the monsters holds up the dungeon. Dead monster, and the dungeon (or dungeon level, or sub-level, or section) collapses. The collapse can be gradual - like in the movies - or sudden and complete.
A dirty, rotten and pretty unfair trick is to do this with no warning. It's also not so productive - it's more gotcha than clever trick -"You acted like I've rewarded you for acting in this game, and now I'm punishing you for acting that way!" On the potentially lethal scale of "drop the world down around you" that's a bit much of a surprise.
Better, and more entertaining, is to let the PCs know that killing the monster will drop the dungeon down around them. The signal can be a shake every time the monster is injured in any way (or one of them is killed, if it's a group of monsters.) You could provide the information in the form of rumors, or Hidden Lore knowledge, of Area Knowledge. It might be written on the nearby dungeon walls. Or perhaps the monster itself will tell them ("Bwahaha! You cannot slay me or the tunnels will collapse and seal your dooooooooom!") It's a great time for a monologue, and they might even listen a little bit if they hear "Let me finish talking or you'll kill your own PCs with your lack of knowledge." Sometimes they won't care, but at least the information was out there.
You could use this concept to create monsters better foiled or avoided than destroyed, in order to get at their loot. If killing the monsters starts a countdown clock, it's better to loot first or you might have no chance to loot later. Putting a tempting bit of loot, protected by a load-bearing monster, presents a choice - take the risk for the loot, or the safer option of just killing it?
While you could do this with very tough monsters, it could provide a violent way out - beat it up, even really unload on it, as long as it doesn't die. A twist on it is a very fragile monster - one that's dangerous to the PCs but which is also extremely vulnerable to the PCs attacks. That makes any aggressive action against the monster(s) risky.
Why does it collapse? - Good question. Magic is the key here - something magical is keeping the level intact. This can be magic-magic ("a mad wizard did it!") or holy magic (the monster is a divine being, tasked with guarding something).
It can occur due to a specific Wish (when I drop, drop my freaking castle on the bastards that killed me!) or because the boss has built a who castle from magic held together with Wishes that won't last after you're gone - or that are sustained due to its personal magical power. These make great sense for mad wizards, mystic wish-granting creatures like djinni, or the like.
This can work very well with clearly magical levels - glowing hallways of pure force with a great treasure at the center, say, or magical dreamscapes inside the very mind of the monster (careful, damage the scenery too much trying to escape/find the loot and the monster dies, and you collapse with the dream), or shadow castles inhabited by shadow beings.
There are a few ways to play this.
Speed of Collapse: The speed is important. A gradual drop means the PCs can flee, movie hero style, after defeating the bad guy. A quick drop means they're just crushed or trapped or killed if they push the monster too far and kill it.
Single Vs. Groups: Is it one monster you need to kill to drop it, or a group that must all be killed? Does each monster in the group drop a different section of the area in question?
Higher Stakes: One of the tidbits I like about the ancient Egyptians, besides signing the insides of the pyramids, is the concept/god of Ma'at - a sort of universal stability. Pharoah needs to be protected or this stability will be damaged - guarding the tomb is important because the stability of the universe is at stake. It's hard to imagine Valhalla holding up well if you whacked the gods, either. It doesn't need to be the question of a single level, it can be the question of an entire dungeon, entire region, or entire world.
It's rough to make a monster hold up the world and expect the PCs to deal with it without violence. But it can be the basis of an interesting reversal:
Save the Load-Bearing Monster! This is just a classic video game mission - protect the hapless NPC/captured enemy spaceship/base from waves of monsters. This just turns turns the MacGuffin into either a Local Load-Bearing Monster (dungeon falls on you!) or a Universal Load-Bearer (reality falls on you!) and the Universe is where the PCs keep all of their stuff.
Have you ever used a load-bearing monster? I haven't, yet, although I've placed a LBM in my dungeon. Either way, I see all sorts of possibilities . . .