D&D 5E Share Your Meanest, Most Dastardly Trap

[Nebulous]

I designed one recently where a huge boulder drops from the ceiling on an area of the dungeon that has a declining floor. It rumbles along behind the PCs, picking up speed as the corridor turns in upon itself in a spiral before ending at a pit which drops into an underwater maze. The corridor through which the PCs are likely running contains areas of difficult terrain, a slippery grease trap, grappling zombies, and a flame jet trap. (Did I mention the grease makes the PCs and the giant rolling boulder flammable?)

Those PCs that can stay ahead of the boulder despite all the obstacles in the way end up at the center of an unlit maze that is completely underwater, floor to ceiling. At some point, that rolling boulder from the level above falls through the pit and, given the dimensions of the chamber at the center of the maze completely fills the room. This potentially pushes the PCs through one of four doors in the chamber, splitting the party. Now it's a race against time to find the way out of the maze before the PCs' drown. This entails finding a sealed door that requires some effort and skill to open. Along the way, the PCs may run afoul of a ghostly sea serpent that can phase through walls.

That is badass and pretty damn mean! I like it.

 

[Herobizkit]

I also used a Reverse Gravity effect that throws a PC up a hundred foot chute onto some spikes. The spikes then retract, ending the Reverse Gravity effect and opening a hundred foot pit with spikes below him. The hapless PC then falls 200 feet onto some more spikes, wherein the lower spikes retract, the spikes above re-appear, and the Reverse Gravity kicks in again...

 

[Reynard]

I also used a Reverse Gravity effect that throws a PC up a hundred foot chute onto some spikes. The spikes then retract, ending the Reverse Gravity effect and opening a hundred foot pit with spikes below him. The hapless PC then falls 200 feet onto some more spikes, wherein the lower spikes retract, the spikes above re-appear, and the Reverse Gravity kicks in again...

I like this one except I would put it in the pit only and have it permanently close after the victims fall the first time so there is no chance of escape.

 

[Ovinomancer]

Hobgoblin keep on a cliff above a river. There's a small dock with a fortified entrance at the base of the cliff. Instead of a frontal assault, the party forces the riverside entrance. Instead of stairs inside, there's a circular ramp with pulleys mounted into the walls. Party immediately assumes the slope instead of stairs is so supplies can be pulled up the ramp using the pulley mounts. So, they don't consider other reasons until the hobs dump the oil reservoir at the top of the ramp, seal the metal doors, open the vent flue, and activate the ignition mechanism. Party first gets slippery oil washing down, a moment of shocked realization, and then a roaring sound as the approaching flames roar down the ramp.

They survived, due to some quick thinking, but after the fire gutter ed out and they ran back up the ramp, they were stymied for a few moments by the still superheated metal doors at the top.

Another was a set of very nasty traps lining hallways. Obvious oversized glowing metal hammer sculptures mounted opposite each other in hallways. They tripped one with a summoned animal and watched it get vaporized by radiant energy. Looking down the hallway, there were four sets of the hammers. A thorough search, however, turned up an off switch concealed on their side of the trap. No such switch was found on the far side. Then they found another corridor, similarly trapped, but with an automatic ballista contraption set to fire down it. Again, the party was on the side of the trap that had the off switches, and the ballista was set to fire in the direction they were going. It was about this point that someone asked, "um, what are these traps trying to keep in?!"