There's a mold on the stairs. My fire imp knows it's there, but it cannot be seen, and so the poor imp's fire bolts hit nothing but empty air...
This is actually a slightly tricky problem, in that I can't go into the wilds while poisoned, and the unkillable mold always hits me when I get close. Hydra breath didn't do anything, but my stone golem helped me out in the end: a stomp re-arranged the dungeon. I've no idea where the mold is now, but the stairs are somewhere different.
Of course, this leads to strange effects upon re-entering the level...
