Locating the Hidden Chamber
Return to Helheim with Atreus and proceed along the bridge destroying the Shatter Crystals. Behind the third and final one is the open Hidden Chamber along with a Raven and an Artefact.
Exploring the Hidden Chamber
Ride the elevator down to the bottom and activate the Mystic Gateway. Climb the stairs up to the top to find a coffin with some resources as well as a Raven perched above it. Turn to the left of the coffin to find an Artefact behind the brazier. In the central chamber, the Valkyrie Rota awaits.