The Prism realm used to seal Satoru Gojo is the final remains of buddhist monk Genshin highlight

What you need to know about Kenjaku.....
Kenjaku’s Innate Technique which has not yet been named allows him to transplant himself into a corpse and thus take over the corpse, along with all of the corpse’s powers and memories. This is how he’s been able to survive for such a long time, and this is also how he managed to take over Suguru Geto’s body once he’s been killed by Gojo in Jujutsu Kaisen 0. He did it because he wanted to get his hands on Geto’s Cursed Spirit Manipulation technique.