The Story of Thresh
Thresh was a mortal warden who worked for the Shadow Isles before the Ruination — before the event that destroyed the islands and created the Black Mist. He was not a good person even then; he was a man who found suffering interesting in ways that he organized his professional life around. The Ruination transformed most people into unwilling undead. Thresh became something else: a conscious, purposeful evil that the Mist amplified rather than simply consumed.
He collects souls in his lantern. He is specific about which ones — he wants people who matter to someone else, people whose absence creates maximum suffering in the living. The lantern is not a simple prison; the souls inside it are aware, and their awareness is the point. He feeds on the interaction between the imprisoned and the people who miss them, harvesting the grief and anguish that this separation produces. He is a craftsman of suffering.
He has a particular interest in Lucian and Senna — the partnership that he disrupted when he took Senna, the partnership that reunited when she escaped, the partnership that now actively hunts him. He finds this gratifying. He designed it to be gratifying. There is something about being correctly understood as the enemy that he experiences as the most honest form of appreciation. He will be patient. The chain is very long. It reaches wherever grief does.