Stealing a beholder’s soul [closed]

So I kinda half-assed a session yesterday where the party found a unscathed beholder skeleton which was feasted upon by gas spores and made a town sick. I just have this idea that the beholder had its soul stolen by some monster or was put in a phylactery. I’m thinking a night hag but I can’t think of a way to implement it or any of the other options.