Apocalypse is a mutant who was born thousands of years ago on the planet Akkaba. A hideously ugly baby, he was abandoned as an infant. He was rescued, but it is not a a happy, feel good story. His rescuer was Baal. He gave the baby the name Nur and raised the boy to be a survivalist. This proved to be helpful, because both the man and the boy were trapped in a cave collapse accident which killed Baal.
This is where things got bad for Nur. Seeking revenge for his father's death required that Nur travel. Partly due to his emotional state regarding his father, and some other events that happen to him along the way Nur discovers the scope of his mutant powers. After making himself over into a super bad dude, and renaming himself Apocalypse, he decides to hibernate until a more suitable time.
When he comes back to joint the living in the 20th century, he often battles the X-Men, because although they are mutants, too, Apocalypse hates their peaceful ideology. He is a very bad force to be reckoned with until the 39th century when his body finally begins to slow down. Then he casts his consciousness into other people's bodies.
Thanks to Jenny for hosting this fun link-up!