In addition to
Smoker4's
node about the
X-Men aspect of
Apocalypse, here is a supplemental
biography of
Apocalypse.
One of the most vile, powerful, and long lasting villains in the Marvel Universe. Of all the enemies that the Marvel heroes have faced, no one has caused more peril, more destruction, more time lines to form, than the mighty Apocalypse. Likewise, no other mutant in the Marvel Universe has created, spawned, trained, or manipulated more mutants than any other. Mr. Sinister, Professor X, Archangel, along with half the known mutants in the world can trace themselves back to good old Apocalypse.
When Apocalypse first appeared he was just another X-Villain, with some crazy plan to conquer Earth and a group of evil mutants that served him. In that time, he was X-Factor's enemy, but slowly he turned into one of Marvel's most fearsome villain. His first big action was the change of Angel to the Angel of Death (Archangel), one of his Horsemen. Since then, En Sabah Nur has proved that he will do anything to assure his survival of the fittest dogma, which he gained from his childhood in Egypt. Apocalypse was born on a nomad tribe in the middle of the desert, and was later kidnapped by another tribe, a vicious one, called the Sandstormers. After finding the chamber of the Pharaoh Rama Tut, that contained time traveling equipment, Apocalypse was kidnapped by the Pharaoh and almost killed. However, by using his powers he escaped, and began to put in action his plan.
It has been revealed that Mr. Sinister was transformed into what he is by Apocalypse and that Nathan Summers, a.k.a. Cable was "built" by Mr. Sinister to kill his master. Another being that calls Apocalypse master is Ozymandias, an ex-vizier of the Pharaoh Rama-Tut. He is now a being of stone that marks all of humanity's history to Apocalypse.
Also, it is known that in the future, Apocalypse ruled with iron hand, and that is the reason that Cable is on our era: to stop his dark future from coming. Redd and Slym, a.k.a. Jean Grey and Scott Summers, with young Cable, have killed Apocalypse in the future, but everyone knows that he keeps returning, to cause havoc and pillage the world as he sees fit.
In fact, he did return during the Onslaught Saga and is probably planning something against the unfit mutants in the world. His last appearance took place in Egypt, when he sent Ozymandias and Caliban, his servants, to warn the world of his return.
A side note of a personal nature, Apocalypse was always my favorite villian in the X-Men Universe. He was always coming back for more, always trying another plan for world domination, and if carried out, his plan for the world would make for an interesting spinoff comic series. Also, my favorite verbatim is taken from one of Apocalypse's wise wise dialogue with Mr. Sinister; "I am the rock of the eternal shore. Crash against me . . . and be broken."