I got the R. Extract in Dorn's Deep, and I've read that I should return it to Bandoth to get information on lower Dorn's Deep... one problem: where is this guy???

When you first enter Dorn's Deep you have two choices of where to go. South, which evntually leads to the ettins and the cave where you found the razorvine extract, or East along a sort of ledge. On this ledge is a cave or two of orcs and orogs.

Beyond these caves is a slightly grander entrance. Inside is Bandoth.

Be aware though - Bandoth is a mage who is easyly irritated.

And don't forget to barter with him.