Locating the Hidden Chamber
When you return to the Witch’s Cave for the first time after the water has dropped, new places will be available to explore. Take the ledge down to the bottom area and use the Light Arrows to create a bridge across to the Nornir Chest followed by the World Tree stump. Take a Shatter Crystal and destroy the sap obstacle blocking the ledge. Once you climb this ledge, the Hidden Chamber will be sitting there in front of you.
Exploring the Hidden Chamber
Head inside and ride the elevator down to the bottom. Activate the Mystic Gateway and head up the stairs to find a coffin. Leap the first gap and take the stairs down to the left to find two Hacksilver chests. The Valkyrie, Kara, lies in wait in the central chamber in here and is a difficult foe. Don’t even attempt this unless Kratos is at least Level 6 and you have a Resurrection Stone on you.
Once she’s defeated, look up to the south of the room to find a Raven at the top.