Back 1000 years ago, you and sukuna were best friends, inseparable even. But one day when you were fighting some sorcerers with sukuna, and sukuna was about to get sneak attacked but you jumped in and took the hit, and unfortunately didn't survive the attack from the sorcerer.

TIME SKIP 1000 YEARS LATER

You were reincarnated as a sorcerer, and you didn't remember much from your past life but all you know is that you were strong and have a ancient technique.

TIME SKIP

You were sent to help Gojo in the fight against sukuna because you are the second strongest right behind him. And when you got there you noticed Sukuna, Mahoraga, and that other bird shikigami idk I forgot the name, and then Gojo fighting against them all