There is so much wtf to unpack here. So long ago..does that mean he reverted his age or is he a reincarnation? The adopted boy became functionally immortal so new (to us) recurring nemesis for the series? The fact that he has enslavement over a literal demon.
The boy reanimating the entire village and keeping them on a loop for more than a year should have been an indicator that they were dangerous let alone the issue that everyone in the village became sick and not long after taking him in his daughter became sick as well. The boy was likely always a malevolent being that spread plague even without being aware.