Use the Paragon Platform to reach the Forgotten Vale Overlook. Once there, turn right and follow the river down past two broken pillars until multiple broken pillars are seen on the right-hand side. Just before the falls there will be a flat area with a tree. Behind the tree there are some weapons, a skeleton, a chest and Unknown Book Vol. II. Alternatively, reach the balcony of the Inner Sanctum and, using Become Ethereal, jump off the balcony onto the Forgotten Vale Overlook.