Destroy the Altar of the Tides with Catelyn's Blade, kill Gazban and bring the Stone of the Tides and Catelyn's Blade back to Baron Revilgaz.
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!