Ok, now I've joint the forum.
I also didn't realize that this demon will follow me. When he first killed me (in larsha) i loaded again and tried to walk to another place. Then suddenly the demon apeared there and so I get to know this. In fact, it was only luck, that I found the solution to this quest.
In the scriptorium I found this "Prince of Persia" gate, but when I walk through this gate, I'm back on the worldmap. So it must be a bug!
I think I have to play the whole game again!