In all of the games, Magatsu-Izanagi doesn't have a special formula meaning a two or three-piece fusion is enough to get him. In Persona 4 Golden, he can only be fused after finishing Adachi's Social Link, so you are required to do that first. In all the other games, Magatsu-Izanagi is DLC, so do be sure to download it before you hop into the velvet room.