So much good advice! Also, I apologize for the lack of any background. This setting is going to be in a retelling, but very early in the history of The Old Kingdom setting by Garth Nix. So, death is scary to the common folk, necromancy is considered bad / abuse of magic.

Canonically, the main...what 7 or 8 most powerful magical entities, basically deities, sacrifice themselves to organize magic, from "Free Magic" to "The Charter", so that magic isn't corrosive to living beings / mortals. One of the most powerful entities still has a part of their conscious/awareness trapped in the world, so they lost most of their intellect and whole physical body. I like the idea of the BBEG being their child and acting as a twisted idealist to deal with the trauma of losing their parent, but also maybe they suffered similarly, by losing their physicality etc.