dnd 5e – How can I create a character possessed while remaining consistent with AL?

I've always liked the idea of ​​possession, so I thought that if I made a character possessed by something. How can I remove this without breaking AL rules?

My initial idea was to redefine the text of the warlock class. I do not know if this is allowed under LA rules. The goal is that the character is owned by another entity and that the entity acquires control of the character as it is leveled.

For reference, I had never played D & D and I currently borrows documents from a colleague. I currently have the PHB, the Volo Monsters Guide and the SCAG on hand.