King of curses? Sukuna? King of demons? Muzan? Neither of them were on Koroshi’s level, The GOD of (all) FIENDS, he had existed for decades. Koroshi was summoned every 666 years to earth. Today was his 2666 summon(birthday) He possesses both Fire and Blood Manipulation, combined these element abilities create the strongest power only he can do: Decay (at will he can make one die by just a touch of his palm) he’s a god, intelligent, and actually righteous.