ToME: the Tales of Maj'Eyal

Well, instead of typical old forest wood i've got weird castle there

Any idea how to finish quest?

I assume you've checked all around the outside for the tree?

If you can't find him, you don't actually need to do that quest to progress - you can go straight to dreadfell for that part of the story whenever you want.

Yep, checked everything.
Thnx for the tip, i will just go to dreadfell

Have got similar castle in 4 lvl of Old forest and the boss was actually in the previous level. Maybe it is the same thing for yours.