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The Demon of Hatred is an optional boss found during Sekiro's endgame. Useful prosthetic tools for the fight include the Malcontent for stunning the boss and Suzaku's Lotus Umbrella for blocking fire attacks.
The best way to fight the Demon of Hatred is by staying close to it and its right side. Because of its size, many of its deadlier attacks are prone to missing you, especially if you dash constantly. Focus on attacking and watch out for trickier moves, like its sweep. It will be a hard fight regardless of your ability, but keep these tips in mind and you'll take the Demon of Hatred down eventually.