Classic Pendulum Trap: Large sharpened pendulum blades swing down from hidden slots in the ceiling. Each character in the area rolls 1d4 to see how many have a chance of hitting them (AC/Dex roll/Saving Throw/etc. to avoid). Each successful hit does 2d6 damage. Optional: There's a 50% chance of poison for each successful strike, with character(s) losing 1 HP per minute until cured or dead.