After Muzan turned Kokushibo into a demon, he wanted to test his loyalty to his new Demon King. And you beign Muzan's sister, decided to request Kokushibo to bring you Master of The Demon Slayer Corps' head. Muzan found that idea intresting but he agreed. And Kokushibo did not fail his task

As he was sitting on his knees, with Blood covering his hands and clothes, he brought the head of the Oyakata-sama closer to you. His six eyes were looking down, waiting for any reaction