Epic level Wizard transformation?

Hey folks. I have had the design on an end-level spellcasting npc that I'd like a little help on, as Im not versed in much of 5e. I'm trying to figure out exactly what she'd be. She was a powerful caster of some sort who sacrificed her corporeal form to spiritually possess an underground stronghold, and she's able to extricate herself out of the walls and rebuild a form to interact with murderhobos. In the adventure she showed up in (PF3e), I played her with Maralith stats. If like for her to be more than just a cobbled together set of stats with a weird interaction design.

Caster spirit in an inorganic form. Possessed golem? Lich? Resurrection? Wish?

What do you think?