Contained inside the Legendary Chest

After grabbing the treasure map, keep going to the right and you will reach a chain. You can ise it to lower the gate, but first destroy the big rock blocking youur view. You have to use the Draupnir Spear for that. Then throw your axe to the gear on the side of the water wheel. Freya will need to freeze it. Then turn back to the other side, and call back your axe. Use it to freeze the geyser, this will lower an elevator. Take the elevator using the now available hook before it. Once on the elevator, use the Draupnir Spear to break the stones that where blocking the gate. Call back your axe to get to the upper level. There will be some wretches to fight. Use the two climbable spots to reach the highest level, and turn right to find the Legendary Chest.

God of War Ragnarok Map

GoW: Ragnarok Map Key