This Saturday the SciFi Channel will broadcast a new Highlander made-for-TV movie. I'm looking forward to this one because it stars Duncan McCleod, from the TV series, whom I always liked better than Connor. When the HIGHLANDER program had its original run, it was fun to speculate on the origins of the Immortals and the reasons behind their powers and vulnerabilities. For example, what would happen to an Immortal buried alive or left to starve in a desert? Would he keep dying and reviving over and over, indefinitely? How long would he stay alive after each revival? Why are Immortals sterile? And who says there can be only one? How did this lore begin, and how do we know it isn't a deliberate lie or a baseless legend? If Duncan and Connor had been the only remaining Immortals, would they have fought to the death? Suppose they didn't want to—who or what could make them? The only origin story in Highlander canon was offered in the abysmally dreadful second movie, which I prefer to pretend never happened. (I don't think many fans of the series acknowledge that disaster as canon.) The "Immortals as aliens from another planet" plotline was not only hokey but illogical. Extraterrestrial invaders have spent millennia seeding Earth with "orphaned" babies? (If I remember correctly, the whole foundling children motif was ignored in that film.)
In my opinion, the clue to the secret is found in Duncan's ostracism from his clan as a "changeling." If the Immortals are in fact fairy changelings (most cultures in the world have some analogue to fairies in their folklore), their unique traits make sense. I like to imagine that they are hybrid children of fairies and human lovers. Unacceptable in the society of the fae, these babies are cast out to be adopted by human families. That explains why all Immortals are foundlings of unknown parentage. Their elvish heritage accounts for their near-immortality, with decapitation being the sole means of killing them. (Yes, they also might be thought of as vampires. After their first death, they cease to age, and decapitation is one of the traditional vampire-slaying methods. Also, they drain energy from their own kind by killing. However, I'm not going in the vampire direction right now.) The fact that they are hybrids explains why they're sterile. What about those lethal longswords they seem to have at hand all the time, yet nobody ever notices? Fans have theorized that the swords are stored in a pocket dimension known as “katana space.” :) Obviously magic! As for the "there can be only one" lore, I think that was promulgated by malicious fae who loathe and fear the hybrids and therefore want to encourage them to kill off each other. Someday I might write a story along these lines, although of course if I don't want to make it Highlander fanfic, I'd have to file off the serial numbers (to quote a favorite saying of Heinlein's).
I've always enjoyed fiction that provides explanations for its fantasy motifs, whether magical or scientific. Magic should have rules and rationales, just as science does. I especially like scientific, or at any rate systematic, explanations for traditional folklore monsters such as vampires and werewolves. I much prefer a werewolf who's born that way, because of evolutionary forces that have created a human-beast blend, rather than one who is "cursed." Jack Williamson's classic DARKER THAN YOU THINK offers an enthralling "unified field theory of the supernatural" (as I think of it) to explain witchcraft, psychic powers, werewolves, and vampires in terms of a near-human subspecies that split off from Homo sapiens in prehistoric times. No pure specimens of this strain exist today, but their bloodlines have spread throughout the human race, some people carrying far more of the "Homo lycanthropus" genes than others. Many authors have created detailed explanations of vampires as another naturally evolved species, such as Suzy McKee Charnas' THE VAMPIRE TAPESTRY, George R. R. Martin's FEVRE DREAM, Jacqueline Lichtenberg's THOSE OF MY BLOOD, Elaine Bergstrom's SHATTERED GLASS, and Octavia Butler's FLEDGLING. Vampirism as an infectious disease is meticulously explored in Richard Matheson's classic I AM LEGEND (which will have a new movie adaptation released this winter), Dan Simmons' CHILDREN OF THE NIGHT, and Scott Westerfeld's PEEPS. Arthur C. Clarke famously told us that any sufficiently advanced science is indistinguishable from magic. Likewise, any creature with strange enough traits appears, to casual observers, a supernatural entity. Think of all those "deities" encountered by Captain Kirk and his crew, only to turn out to be super-powerful aliens with inflated egos.