Locating the Hidden Chamber
Once you enter the Heart of the Mountain, look to the right of the area to spot the large door with the lock on it.
Exploring the Hidden Chamber
Take the elevator down to the bottom and activate the nearby Mystic Gateway. At the top of the stairs you’ll find a coffin along with a Hacksilver chest and a sack of Hacksilver. Head back downstairs and leap the gap to find a second sack of Hacksilver to your left and a second Hacksilver chest to your right. The Valkyrie, Eir, awaits in the central chamber. Upon her death, look above the brazier in this room to find a Raven.