Phil is immortal to death from natural causes. If he is killed directly by a mortal being that day starts over again and no one except the murderer is aware. Every time this happens it creates a branched mirror universe where the main time line is the one where the mortal decides not to kill Phil. Those branch universes don't last forever though, the people inside of them rot and die painful deaths as the force that sustains that universe runs out. They shouldn't exist and therefore will stop existing in weeks, months, years, or maybe decades.

The souls created in these temporary universes of pain live on and ultimately find their way to the main timeline to unconsciously torment the one who killed Phil. A curse that is passed down the family bloodlines and to close friends causing untold suffering. From sickness, to bad luck, to the shadows that move out of your eyesight late at night, the pain is shared. Eventually all the living people who were not split from the main timeline die and whatever eldritch force that holds that charred duplicate remains of their lost soul, dies with them. Once again the family and friends of the one who murdered Phill will know peace.

So think twice before messing with Phil.