Magneto and Professor Xavier have one of the most complicated relationships in comic book - and film - history. They're the worst of enemies, and the best of friends, often in the space of one story. They're equally passionate but their ideologies can be poles apart. It's a big part of why the X-Men movies have been thrilling audiences for over 15 years.
There's a lot going on in X-Men: Apocalypse. Not only does the super-villainous mutant of the title want to destroy civilisation as we know it, but Quicksilver will come to the fore in his mission to form some kind of father-son bond with Magneto, whilst Mystique leads the charge against Apocalypse in a bid to recover some of the morality she lost a grip of in Days of Future Past.
But among all these character-driven story strands, one will stand out: the way in which the dynamic between Charles Xavier and Erik Lehnsherr makes its way toward some kind of conclusion - and as endings go, there's every chance this one won't be very happy.
In the comics, Charles has a pretty black back-story: prior even to begin born, he becomes a murderer after sensing his twin brother's evil and using his cognitive abilities to kill the second fetus before it can enter the world.
In many ways, this fact of Charles' existence plays a big part in his dynamic with Erik, the two men's relationship more like that of brothers than friends - which is why, despite their inherently different attitudes towards the survival of the mutant race, they can never fully break their ties and fight on opposite sides.
The alleged trailer breakdown
This week an alleged description of the first X-Men: Apocalypse teaser made its way across the internet - here's a condensed version:
The trailer opens with Jean Grey waking from a nightmare. She tells Professor Xavier something along the lines of, "I have seen the end of the world. There was only death." There's a montage of footage of the young mutants at Xavier's school, followed by a scene of Moira MacTaggert telling Xavier that they've discovered a god-like mutant who may be the origin of the species.
Magneto asks, "Who are you?" The figure pulls down his hood to reveal the face of Apocalypse. There's a montage of the destruction caused by Apocalypse. We're shown two opposing groups of mutants (likely Apocalypse's Horsemen and Xavier's students and/or allies). Mystique tells Xavier, "I came to fight."
Now, I'm not actually sure this is legit, because it seems unlikely that anyone who's seen it would be spilling the tea on the internets, but who knows? One thing's for sure - it does confirm what we already know about the division between Mystique and Charles' mutants and the four Horsemen.
The legacy of Apocalypse
It's no secret that Apocalypse wants to change the face of the world, for reasons as yet unknown, but which will hopefully be revealed soon enough. Whether against his will or by persuasion, Magneto becoming one of Apocalypse's Four Horsemen - and going to war with Charles and his students - is going to drive the wedge between the two further, and deeper than ever.
The questions the film will therefore ask - how far can two opposing philosophies be stretched until no mutual ground can be found? - will likely strike a more intelligent tone than two similar films pitting hero versus hero, Captain America: Civil War and Batman vs Superman. In those films, the prospect of a big fight between the two protagonists is in such heavy focus that the sense that the fight is earned, that the two characters share enough love to hate so intensely, is lost. In Apocalypse, it's earned. These two men have six films worth of history.
Which is why Apocalypse's biggest legacy, for better or worse, could well be the effect of his crusade on the brotherhood of Magneto and Xavier. This film will either bring the two men together in the name of a common cause, or tear them apart for perhaps the final time.
And I can't wait.