Found in a strongbox in the Cave of Hollow Time together with the key needed to leave Coldharbour. The strongbox is to the left of the boss chest. Cast Ancient Vision to see it. The Cave of Hollow Time is located south of the Coldharbour exit by the beach, near a pillar and a brazier. In order to see the entrance to the cave, cast Ancient Vision.
Requires The Gray Cowl of Nocturnal.