The party just decided to venture to Undermountain. They're now in a room with several pillars and four corridors. While one is leading to where they're supposed to go, two are not. In effect, one is the entrance to the realm of a powerful wizard that doesn't really want his domain to be disturbed. He wrote some warnings on the wall: "danger this way", but the PCs being PCs, they decided to go that way.
When they're gonna head that way, a...