In the harsh, unforgiving desert of ancient Egypt, ruled by Rama-Tut, a band of nomadic raiders found an infant, gray-skinned and freakish in appearance, abandoned by the settlers of Akkaba. The nomads took the child for their own, giving him the name En Sabah Nur, “The First One,” and teaching him to be "strong" in order to survive the desert as a child. This idea, that "the strong will survive," would shape Apocalypse's actions throughout time.
Apparently, Apocalypse would use Rama Tut’s technology to "regenerate" for long periods of time, becoming more and more powerful when awakening. He was also known to have traveled around the world during these ancient years, appearing to various primitive cultures as their death god. At some point, when traveling through Mongolia, Apocalypse encountered a spaceship abandoned by the planet-judging aliens, the Celestials. His travels then became fueled by a single-minded purpose-- to find the key that would unlock the secrets to this alien technology. He found it in the person of Nathan Summers, then wandering various eras as the Traveler. The two fought, and although Apocalypse was nearly killed, he was saved by Ozymandias and taken to the Celestial ship. Apocalypse’s blood had become infected with Summers’ techno-organic virus during the battle, and the infection allowed Apocalypse to not only transform into an even more powerful being, but to also interface with the Celestial ship, claiming it as his own.
Another reawakening occurred during the 12th century Crusades. Apocalypse's regeneration chamber was thought to be a temple of great power, and was sought out by the Black Knight and his friend Bennet du Paris, whom Apocalypse recognized as a mutant, and created him into the being known as Exodus.
Later, Apocalypse surfaced in late 19th-century England, about the same time Darwin was proposing his "survival of the fittest" theories. Apocalypse, searching for underlings, met the scientist Nathaniel Essex, whose own desire for scientific knowledge led him to foresee the abundance of mutants that would occur in modern times. Apocalypse earned the allegiance of Essex, turning him into the being now known as Mister Sinister, but was soon defeated by the time-lost mutants, Cyclops and Phoenix.
Apocalypse abandoned his Horsemen in favor of his Riders of the Storm (a.k.a. the Dark Riders) and later became the target of time-traveling Stryfe, who, as a clone of Nathan, was raised by Apocalypse in a horrific alternate future in which Apocalypse had risen to power. Eventually, Stryfe left Apocalypse withered and dying on the surface of the moon, where he was confronted by his Dark Riders and left for dead.
Later, Apocalypse tried to revive his team of Horseman, transforming the Hulk into War, until Hulk broke free and went his own way. Over time, Apocalypse managed to re-create his Horsemen, transforming recognized heroes and villains into versions of War, Pestilence, Famine, and Death, (specifically Deathbird, Caliban, the X-Men’s associate, Rory Campbell, and Wolverine.)
Soon, Phoenix and Cable left the X-Men in order to focus their attention on a search for Summers, whom they believed to still be alive. They tracked Apocalypse to his birth place in Egypt, where Summers, now an amalgamation of himself and Apocalypse, fought them. Ultimately, Jean was able to physically rip Apocalypse from Summers' body using her mental powers, and Cable destroyed Apocalypse's essence with his own telepathic powers.
More on Marvel.com: