Seeking revenge after his wife Mary was killed by the demon Azazel, John Winchester became a hunter and raised his two sons to fight the supernatural. John disappears early in Supernatural, and the first season revolves primarily around Sam and Dean trying to find him.
Goading his son to lash out, the phantom Lucifer needles his way into his son's psyche, creating a spiral that eventually leads to Jack panic-killing Mary so she wouldn't tell Sam and Dean about him misusing his abilities.