Locating the Hidden Chamber
You can’t miss this one during your first visit to Thamur’s Corpse. Once you dock from the boat, run forward and the Hidden Chamber is in plain view.
Exploring the Hidden Chamber
Inside you’ll find a Jötnar Shrine to the left of the entrance before taking the elevator down. At the bottom, open the Hacksilver chest to the right of the Mystic Gateway and find a coffin at the top of the stairs. One more Hacksilver chest awaits to the right once you leap the gap and you’ll find the Valkyrie [Gunnr] in the central chamber. Upon her defeat, look up at the tree to find a Raven perched there.