So you're looking for an altar of some sort? Well, I'm no expert on troll ruins like my father, but I have heard about a sunken troll city out near that island you're looking for. Maybe you should try there.
You got my knife back for me, so I guess you can borrow it... but I want it back when you're done!
Amazing! The Stone of Tides! It shall have a place of prominence in my collection of ancient troll artifacts. Did you see how it almost seems as though a tide is trapped within its surface? Magnificent!