I’ve seen a lot of nasty traps in the old Grimtooth’s Traps volumes, but I think I may have found the worst trap ever. In the Rat on a Stick module, there’s a trapped room. After the first two PCs enter, a door slams shut and inside a voice booms out that only one of them may leave alive. The door will not open until one PC has slain the other. There’s something so fiendish about making a PC do the trap's dirty work.
So, what’s the most diabolical trap you’ve seen or experienced?
THe Tom of Horrors with the flase entry and the various traps in the 1st corridor.
Labyrinth of Madness also had a nasty one. It was for level 15+ adventurers. You climbed down a ladder and it had a Glyph of Warding on it. If you took mor than some low amopuint of damage you had to make a strength check with a -10 penalty (DC 20-25 in 5E) or fall and take 8d6 or 10d6 falling damage. After you took the glyph+falling damage you got hit with Power Word Kill which killed you if you had less than 60hp with no save.
Not ehtthis was AD&D 2E. A level 1wizard with 16 con had an average of around 50 hit points, a fighter might have 90-100 hit points at level 15.
Then the trap animated your corpse and levitated it back up the haft to fight the party with the idea that mayber a cleric will turn the corpse and destroy it making resurrection difficult. If I made the same trap now I think I would use 2 glyphs of warding a DC 25 strength check (or DC 30 athletics), and power word kill is weaker now and PCs get more hit points. The 1st glyph owuld knock you off, then you take the falling damage and you land on another glyph which triggers the power word kill.
One can also use old school save or die traps and if you fail your save you are reduced to 0hp then the power word kill could hit you and finish you off I suppose. Traps targeting int and charisma saves could also work, psionic mindblast int save type traps.
The problem with a lot of traps, is that they are 1-shots. And that just doesn't make sense in many situations.
One of my favorite and deadly traps I created was a pit trap of all things. 10 by 10 spiked (or doesn't have to be spiked) pit 30+ ft deep, but here's the kicker... When you fall in, the ceiling above the pit opens up and a geletinous cube or ooze monster (the exact 10 by 10 size of the pit mind you) falls on top of the poor pc, and proceeds to slowly kill & eat the pc.
One of my most memorable traps was a huge room (60'x100' IIRC) with two entrances (both double, inward swinging doors) and a lone capstan in the center of the room (picture the original Conan movie, while Conan is growing up). The PCs took minutes investigating the room and capstan until they finally decided to turn it. Turning the capstan triggered a complete flooding of the room (a reservoir, holding plenty of water, was hidden overhead), which forced the doors closed and kept them closed. The party escaped by using the party wizard (who had been turned into a stone golem) as a battering ram, before the flood waters rose too high. The build up, while the party decided what to do, was the best part for me.
One of the best trap that I used was in a tumulus. At the top of the tumulus, there was a 20 foot deep round pit, uncovered and fairly easy to climb down into. But the bottom of the pit actually opened in a larger chamber, right in the middle of the celing, again 20 feet from the floor. The light from outside could not show the corners of the room, and actually the angle from the top did not allow people to peer into the chamber. The only "trap" part came from the fact that the floor below the opening in the celing was pressure sensitive and when more than 20 lbs was on the floor, it would cause a sliding metal plate to violently shut the bottom of the pit off from the chamber. It would also release skeletons (also works well with golems) from alcoves in the room, in the four corners, with the instruction to kill whatever was in the room (actually render him unconscious, skeletons don't have first aid to check whether an adventurer is truly dead) and dump the remains in a corner of the room, then return to their starting position. As sson as one of the skeletons (or, actually, any weight) returned to one of the alcoves, this would, in turn, open the sliding plate again.
When I ran it, it worked exactly as intended. The party, using a rope, lowered an adventurer through the pit. When he reached the floor, the plate came sliding out, severed the rope and left the adventurer at the mercy of the skeletons, in the dark. he did not dare to move, was killed and removed to a corner. Then the skeletons went back to their alcove, and some of the members of the party who had actually come down to investigate the plate, were dropped again to the lower floor, while those who had remained above could only see traces of blood on the floor from above instead of their former comrade, before the plate came sliding again...
There were more of them, and actually managed to destroy some skeletons, and one of them, running around the room, went into an aclove and opened the pit. The problem is that it drew the others to the opening, which closed the plaque again. After some confusion, and one more death, the survivors managed to understand how it worked and got out on another rope lowered from above and jumping to catch it rather than walking on the centre section.
All in all fairly satisfying... :p
I would add that this was a low-level "designed-to-be-deadly" campaign, in which most players lost a few characters, until the pool of adventurers were proud to have survived and be high-enough level to have some longer term life expectancy. Some of them even managed to become gods by the end of the campaign, 7 or 8 years and 20+ levels later... 