In Dorn's Deep, I get the rasorvine extract.
I know that I had to give it to someone called Bandoth.
But where is he ?

Hamakeis.

When you first enter Dorns Deep theres a path skirting along the north side of the cavern. Its not too noticable but you'll see it. Follow the path until it ends at a weird building, thats where you want to go! Theres a couple other areas to go to on the way there also. Hope that helps, welcome to Ironworks!

I had already tried to enter that building, but my party
was never able to find a path to reach the door.
I'll try again. Maybe this time i'll get luckier.

The path is right after you get in by the mushrooms to the north. It doesnt look like much, try moving your party north as soon as you get out of the entrance tunnel.