Born under the name of En Sabah Nur, Apocalypse is the first mutant to have ever existed. He was born in ancient Egypt and he's a mutant blessed with immortality among many other gifts.
A believer in "survival of the fittest", whenever Apocalypse appears it is not so much about ending the world as it is about testing it to see if both mankind and mutant-kind are fit to exist.
If the world fails to stop him it'll perish as he puts it through the harshest conditions and trials, but if it manages to rise against him, not only has its inhabitants proven their worth in his eyes, but they've also "evolved" in doing so. Satisfied, he'll vanish away, just until the next time he desires to test the world again.
To this end, Apocalypse is a recurrent and natural enemy of the X-Men. Across his appearances in the comics he has gone against a number of opponents, sometimes coming out victorious, other times getting bested, and, on occasions, even fleeing combat. Apocalypse is that baddie that just won't die. Whenever he appears, though, like his name would suggest, the stakes are always high.