This room has a horde of Slime in the NW corner surrounding a "chest" (actually a Mimic) and a Reaper with three real chests behind solid walls in the SE corner. Only when someone stands where the Mimic is (after killing it, of course) do the walls open to let you at the Reaper. Until that time, expect the Reaper to cast lots of Sleep spells. After your heroes have whittled down the Slimes, send two of them to take care of the Mimic, and arrange your other heroes to bring all they have on the Reaper. He should go down in short order.