I finished the quest but the strange amulet will not turn into the Necromancer's Amulet and I can't take it out of my inventory

Welcome to the club. My first build still has this, the attunement sphere, and a book stuck in inventory.

I could've sworn that I sold that thing.

This

Yes, in that quest you have to sell it to get it to change and lose its quest item tag. I always think it's evidence and not to be sold. Kind of not intuitive at all...

It really boggles my mind that they refuse to fix these SIMPLE bugs.