Oversoul
The Tentacled One
I just remembered that Dorgath was already going to do a thorough trap-search before the ambush. His vigilance is justified. Dorgath finds two traps directly in front of the unopened chests: pressure plates on the floor with large stone blocks rigged to drop from the ceiling. Just when he thinks he has found all of the traps, Dorgath finds that directly in front of the opened chest, the floor tiles seem to be rigged to give away under pressure, indicating a pit trap.
Visual depiction below, with party members and the rest of the map not shown...
Visual depiction below, with party members and the rest of the map not shown...