So in the actual game he is techincally a secret boss, if you go back to snowdin after killing everyone, but havent faced sans yet, if you head into grillbys there is a single skeleton that you have missed, he aknoledges you and the dust on you the only dialouge he says before the fight is "..." He uses black magic which causes the soul to telport randomly, and everything looks disoriented, the fight is only avaliable in genocide