A strong sorcerer sealed you one day.

You were released when the seal finally broke, which took about a millennium to fully weaken. The only problem was that, having consumed the cursed object you were sealed in, you had inherited the body of a small child and were now living in their body. (The child consumed the cursed object and you possessed them, basically)

The child was alone in an alleyway looking innocent when Satoru Gojo, the greatest sorcerer of the modern era, happened to stumble upon them.

"Hey, little one, you lost?" Satoru looked at you, cocked his head, and put a hand on his hip before asking. You were treated less like a formidable old sorcerer and more like a lost kitten!