When Bloody Graves are automatically being invoked or summoned, there is an enemy nearby called Biwa Boku-boku that must be killed. He's a bald, pale-skinned demon playing a guitar. As long as he is alive, Revenants nearby will spawn from red graves. Once he's dead, enemies will no longer spawn from Bloody Graves and Biwa Boku-boku will not appear even after using a shrine.