Bitter Rivals (WAR History and Lore)  

On the Order side of the map, southeast of the Ritual of Corruption Public Quest there is a small Dark Elf encampment. Southwest of this camp is a large tree, around 33k/20k, with the body of A Tortured Rival slumped at it's base. Interacting with the body will grant you the unlock.

History & Lore in Avelorn