Towards the end of the game, you will have to travel to the Dragon Shrine to obtain the Ashen Mist Heart from the Ancient Dragon. The Ashen Mist Heart allows you to travel into the dark memories of the past. You can use the key on hollowed giants in the Forest Of Fallen Giants. You must fight the Giant Lord by entering one of these memories to proceed. However, there is also a secret place to use the Ashen Mist Heart. After obtaining the Ashen Mist Heart, travel to the Duke's Dear Freja arena, and a new crystal will appear under the hollowed dragon. Interact with it to enter the Dragon Memory. You will be taken to an arid wasteland resembling the dragon battleground described in the original Dark Souls. Interact with the ancient dragon's corpse to acquire an Ancient Dragon Soul.