luker above, trapper.
edit to add. 10 by 10 room. Lurker above (monster) on ceiling. Trapper (monster) on the floor. Or did they get rid of those in 4th.
Pit trap with an iron-lidded box filled with acid at the bottom. First person to fall in knocks the lid closed. To get out of the acid-filled box they have to lift not only the iron lid, but also the weight of any of their friends who fell on top of the lid.
Best used in a place where multiple PCs are likely to fall into a pit trap one after the other.