En Sabah Nur, or better known as Apocalypse, is one of the main antagonists of X-Men: The Animated Series. He is an ancient and extremely powerful mutant whose ultimate goal is to rule the entire planet and remake it in his own image.
Stating that he knows more of this world than we dared dream, his ultimate goal is to destroy the current world so a new world could be created in his image.
He created a device which would turn mutants into his Horsemen. Eventually the X-Men defeat this attempt. Apocalypse eventually in the future realized that the battle of good vs. evil could never be seemingly won upon facing Cable. In their battle Apocalypse stole a device that would allow him to travel time itself. Apocalypse attempted to go back into the past to eliminate the most powerful psychics only to end up in the center of all time known as the Axis of Time.
There Apocalypse spent centuries learning of the Axis' power where he found he could sacrifice the psychics which would cause time to collapse on itself due to their vast powers and all them dying simultaneously in the Axis effecting the time stream. Apocalypse realized that the Axis and those inside would not only survive but they'd be able to then recreate reality as they saw fit. Recruiting several mutants only letting Mister Sinister and few know the truth. Eventually the X-Men find the secret of Apocalypse's immortality (a device that rejuvenated him every century) and plot to destroy it when it was fist created. With Cable's help and new time machine they succeed but Apocalypse isn't effected since in the Axis he doesn't need the device to recharge him and being outside of time he won't be effected by those events.
Ultimately Magneto and the X-Men are able rescue the psychics before Apocalypse can sacrifice them. After his plan failed, the psychics used their combined might to banish Apocalypse to the astral plane.
Unknown to all Apocalypse while in the Axis saved Fabian Cortez just in case of such an event. From the astral plane Apocalypse demanded Cortez find him a new body so he may transform it into an exact copy of his original to return to Earth. After Cortez failed to find one, Apocalypse used Cortez’s own body and returned to Earth to continue his mad dream.