To distract the Umber hulks .. you'll have to run an invisible party-member through the chamber they're in. Immediatly after you exit that room, you enter a square-shaped room with 4 exits. The one you just came out of, one to the middle north of the room ( troll-king), and two south-positioned exits, each near a corner of the southern wall.
Go through the first south-exit you encounter, leaving the Umberhulk room, proceed downwards untill the hall turns right, to your left there should be a door/gate,..behind that just drop the umnberhulk-food in the hole in the ground ( dont just drop it, place it in the hole ).
Umberhulks are going to be advancing on you now .. so dont go back the way you came, but go to the right instead, exiting though that second southern-wall exit there was in the square-shaped room.
I hope it makes sense .. it's incredible obvious once you see it, but using words to describe it seems to be a bit more tricky
