Tell me about your interesting and unique traps!

step 1:  pick a room with a room above.  bottom room should be less than 10 feet tall and no more than 20 feet tall

step 2: pick a monster, i typically choose a troll but as long as it sleeps whatever monster works

step 3: cut a hole in the floor of upper into lower of sufficent size and insert trap door and attach it to your preferred trigger,

step 4: hide the trap so its harder to spot on both ends

step 5:  place the monster's bedding on top of the trap door.  this is where they sleep now.

effect:  sleeping troll fall from ceiling, angering the troll who will attack whoever it sees when it hits the floor

Opening the door to the room causes a trap to trigger, once all the players are inside, the door closes and locks and the room fills with water and the characters have to push a pressure plate in the floor to open a hatch on the ceiling

Trapper on the floor and another on the ceiling.

Roper on the floor and Piercer on the ceiling.

50 drow elite warriors

A room that starts filling up with acid, that can only be stopped if you kill the Clay Golem in the center. 

https://www.youtube.com/c/WallyDM/videos

I've been making some for my labyrinth, focussing on prolonged traps which need to be beaten before a certain number of rounds have passed.

room filling with water, ceiling descending to crush them, spinning disorienting rooms, gas-filled rooms, water-weirds in ankle-deep water, to name a few off the top of my head.

Vents open from the roof, dropping Black Pudding.

Quote from SqueegeeLord >>

Opening the door to the room causes a trap to trigger, once all the players are inside, the door closes and locks and the room fills with water and the characters have to push a pressure plate in the floor to open a hatch on the ceiling

I tried something similar to this once; the Ranger whipped out Nolzur's Marvelous Pigments to quickly paint drain holes. I did not see that one coming. All the trap really ended up doing was minor inconvenience of mildly soggy boots.

The Big Book of Killer Jokes.

Upon reading the title, the target must make a saving throw vs a Suggestion to read the book.

If they fail the save, they open it to discover that chapter one is Exploding Books.

Room filling up with any liquid (not water) that aquatic races and water breathing spell doesn't allow you to breath in. The oxygen escapes through tiny holes in the roof that only lead out of the dungeon. At the bottom of the dungeon, submerged under the liquid, is a puzzle for the players to solve (up to the DM). 

They have to finish it in 10 real life minutes, or they lose. If they lose, they either die or have to cast gaseous form to escape through the air holes in the roof. 

A room that opens into a long chasm. Painted on one side of the chasm is a mural, and on the players side there is a statue. When the players approach the statue, it comes to life and asks them what the mural depicts. The characters can't make out what the mural is, but lying on the ground there is a telescope. When a character puts the telescope to their eye, a spring loaded needle comes out and stabs them. Possibly reaching their brain, almost certainly partially blinding them. 

There's a book "Grimtooth's Traps" and a sequel to it. Has lots of ingenious and very vicious traps in there.

The traps themselves are system agnostic, but heavily tailored for fantasy dungeon crawls.

Edit: just realized that there was 4 sequels, which made 5 books of Grimtooth's Traps. There is a reprint collection of all 5 out there...

Quote from Lyxen >>

 [...], these books come from a less forgiving era... :D

Are you calling me old?  ;P

(I am -_-)

Yes, Grimtooth's Traps are not for the squeamish, but they *are* "unique and interesting"...

The players walk into a room. It is furnished as normal. The large carpet in the center of the room is really a 10ft deep shag carpet that just happens to also be a mimic. 

Oh come on.

Nobody's going to trust a shag carpet- they'd burn it on principle even if they didn't suspect a trap.

Many of the traps previously mentioned are pretty lethal. Do traps have to be so dangerous? 

Once used a hanging upside down piglet (who was squealing its little heart out) to make a fighter step on a goblin's snare; thus suspending him 10 ft off the ground followed by a goblin hunting party showing up.

I've recently started writing about traps because I personally struggle with them. They end up being underwhelming at my table. I find the most interesting traps are ones that are more like "Dungeon Hazards" and are visible to the party, rather than hidden behind a Perception DC. 

Brown Mold in patches around the dungeon room, where a combat is about to take place. In my case it was a cavern with a Purple Worm and patches of visible brown mold.

One trap I tried was a pressure plate in a cavern hallway that was covered by an Intelligent Black Ooze. The Ooze would move out of the player's path in hope they would step on the pressure plate and die to the (not really lethal) trap so it could eat them.

Traps are more interesting when the party can interact with them outside of just triggering them and suffering the effects. A quicksand pit that the important NPC is slowly sinking into, with Goblins starting to swarm and poke at the party with spears. A narrow walkway over a deep chasm with a swinging blade slicing across it at the halfway point, and enemy archers firing at the party from afar. A set of fire breathing statues in a hallway and a floor slick with flammable oil and barrels of flammable oil just down the hall.