"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

I killed the assassins and returned to the ghost. He said that I could enter one of the portals. In one, his daughter is imprisoned, in another, there are various goodies, and in the third, there are 3 level-up items.

I don't remember if the Ring of Sinners was on Arantir's finger. Most likely, yes, because it's a great artifact.