Locating the Hidden Chamber
You'll most likely encounter this Hidden Chamber at the beginning of your journey, just after the defeat of Dauði Kaupmaðr. Climb the ledge nearby and in the next clearing, the chamber will be visibile at the back.
Exploring the Hidden Chamber
Take the elevator down to the bottom and activate the Mystic Gateway. You'll find a coffin at the top of the stairs and then take the stairs down once you leap the gap. At the bottom you'll find two Hacksilver chests guarded by a high level Draugr. In the central chamber, a Realm Tear awaits featuring a battle against many high level Draugrs and a high level Revenant. In the room following their defeat you can find a Raven circling the tree and a Lore Scroll on the ground.