Watercurrent Ceiling (4e Trap)
From D&D Wiki
This trap is a more advanced version of the watercurrent wall. It will do more damage, and instead of pushing the target, the target is knocked to the ground and pinned in place by this burst of water. The trap will delay until there's a hero under it, in which case it will attack each turn. This trap can be used together with melee monsters, who will take advantage of the immobilized hero, or when the heroes have to escape a room slowly filling with water. (This trap can ensure that the heroes first have to rescue their ally before escaping.) A dungeoneering check DC 22 is required to realize that there has to be water flowing above the ceiling.