Who would have thought that the shadowy figure pulling the strings of a mostly forgotten group called the Alliance of Evil would turn out to be such a persistent and powerful foe for the X-Men? Apocalypse's first appearances in the original X-Factorseries might not have initially seemed like such a big deal, but it was not long before this villain turned out to be someone very memorable indeed.
Obsessed with the idea of survival of the fittest, Apocalypse's motives can sometimes be murky (and don't get us started on defining what exactly his powers are), but there's no question he is a major player behind the scenes in X-Men history. Shortly after his debut, Apocalypse would transform Warren Worthington from Angel to Archangel, forever altering that character's path - something Apocalypse would do several times again in the future with other notable characters.
His other recruits, both volunteers and those he forced to do his bidding, have included Wolverine, Hulk, Caliban and Sunfire, all of whom served as one of his Horsemen at one point or another. Meanwhile, his ever more elaborate back-story revealed he was born thousands of years ago, in Ancient Egypt, and in fact was the oldest and earlier Mutant on earth, having survived encounters long in the past with everyone from Kang the Conqueror (in his Rama-Tut guise) to Dracula himself.
Apocalypse is also responsible for an 19th century scientist becoming another major X-Men threat, Mr. Sinister, and it was Apocalypse's actions that led to Scott Summers' baby son being infected with the techno-organic virus - thus beginning Nathan's path towards becoming the futuristic hero Cable, who would go on to be one of Apocalypse's greatest foes.