- Isamu is the son of Muzan and has been for nearly 600 years, however the core is unaware however hes been a slayer since he was 12, and has been actively supporting Kagaya and his family for awhile - during the arc it will be revealed and everyone will start to shift there gears Tags// #infintycastle #fyppppppppppppppppppppppp #knyoc #knyboy #kimetsunoyaiba #hashiratrainingarc