The source is always magic, but in the case of vampires and immortals I think that what gives more strength to their immortality are the deaths caused to feed, I think the gods would also be considered, in season 4 it is said that the gods punish humans with catastrophes: earthquakes and fires? they said that believing in them keeps them strong, I had a theory, fearing them can also give strength, it's a form of belief too, having fear makes people believe, but I think the death of these various people in these catastrophes also feeds Immortality ( The Hollow used it to increase their dark magic, and Silas used it to maintain their psychic powers), so in a way immortals are freaks because of that detail? are they surrounded by death whether it's feeding: vampires, immortals and sirens? or causing catastrophes: the gods? I think Landon might be a different case too, I think his own death fuels immortality, (in the sense of dark energy/magic caused by death as it was for the Hollow? only in his case it's death itself that gives him energy to return from the spirit world).
