UnderKaisen is a concept that's been around for a little while. Basically just a combination of Undertale and Jujutsu Kaisen. Sukuna is replaced with Chara. (Charakuna) Yuji is replaced with Frisk (Friskadori?) Gojo is replaced by Sans (Sojo lmao) The other characters you can figure out for yourself because I'm lazy. Have fun!