Actually it be closer tn a cross between the two. Pretty much a powerful necromancer brought the character back for reason I shall not talk about here in public. Anyway necromancer brought the character soul back then placed it in a phylactery giving them intelligence and independence. As for how that would work I dunno. Some really powerful spell form the Book of Souls. Maybe its a forgotten spell or some experimental spell.
   

Although if that makes no sense or just a no no I be happy with plan old basic lich.