黒死牟

Kokushibo is the main secondary antagonist of Demon Slasher: Kimetsu no Yaiba and the First High Moon.
Nearly five centuries ago during the Sengoku Period, during the Golden Age of Demon Slayers, Kokushibo was a man named Michikatsu Tsugikuni and a former member of the Demon Slayer Corps and the older twin brother of Yoriichi Tsugikuni, the strongest demon slayer to ever live.