Dark Magic (WAR History and Lore)  

Interact with the Worn Cage located just north of the Mirelen Public Quest around 54k/3.8k to trigger this unlock. The Worn Cage is located within an area with many different cages, all of which can be used, so make certain you are using the correct one for the unlock. Besides being labeled differently, the Worn Cage will not have a prisoner inside while the others may or may not.

History & Lore in The Shadowlands