how did kokushibo die

Kokushibo died during the battle in Demon Slayer's Infinity Castle arc. He was fought by Muichiro, Genya, Sanemi Shinazugawa, and Gyomei Himejima. While Muichiro and Genya were killed, Sanemi and Gyomei managed to decapitate Kokushibo. However, Kokushibo regenerated his head thanks to his demon powers. The critical factor in his death was the burning red blade used by Muichiro, which caused Kokushibo's body to disintegrate despite his regeneration. At the moment of death, Kokushibo reflected on his tragic life as Michikatsu Tsugikuni, twin brother of Yoriichi, and admitted that deep down he only wanted to be as strong as his brother. Ultimately, he lost his will to fight and allowed himself to die, expressing regret for his monstrous existence and his failure to leave a legacy.