Spoke to Xikâsz just before dealing with the guy in the cave (I didn't explore that part of the map and realize he was there earlier). He left without joining the party. I presume he's supposed to show up again, but I'm not sure where to look for him.
I don't understand your question. First, you meet Xikâsz outside, near the cave entrance. Then, you should go inside (the main complex, I am not speaking about the "green man") -- from there, you can either go up and face the gnoll vampire leader, or go right, deeper into the caves. You should go deeper and deeper until you reach the final battle. He'll be there.
Oh, I was too focused on the green man and missed the other cave entrance lol, sorry for the bother.
Hah! No problem! Enjoy the fights! (BTW, you may want to get rid of the mini-boss first -- the vampire gnoll leader, IIRC, there is a very useful book somewhere there)
Cheers!