I didn't talk to the necromancer again after our first meeting, until level 10 when I got the crystal skull (and the only parts available then were the default versions). I didn't check if the third piece got added to the necromancer's list after it disappeared from my inventory (I just reloaded). I guess it is possible the first two pieces I found were the same and worse than the default version the necromancer has? I'll check again the next piece I find (I think I may have deleted the saves with the third piece, clearing out room to be ready for my next play session, because of the stupid 20 save limit).
