Most interesting boss in the game lore-wise.
He's a literal evil tyrant who usurped a god, enslaved the local population, Vordt and the Dancer, plots behind the scenes and sends his agents to other countries to spy, kill you or slow you down. The only one (Sable Church aside) who actually has a plan and ambitions for after the fire faded. Every other boss in the game is either insane, desperate, just wants to survive or an animal that just protects its territory.
Also the hardest boss in the main game and a roadblock for hours, at least for me. A shame he isn't even the main boss of his area and by the time most players killed him, they have no clue who and what he was.