Chapter X : Onslaught and War
While Apocalypse may have slumbered through this tumultuous event that almost changed the world in a moment, the ancient mutant was very much active and in full form during the next crisis that rocked the X-Men. Awakened from his regeneration, Apocalypse was informed by his scribe, Ozymandias, that a new player in the world of mutants had entered the scene; a being called Onslaught. Appearing suddenly out of rumors, the powerful Onslaught moved quickly to establish a stranglehold over Manhattan. Constructing a colossal citadel made of psionic energy in Central Park, mentally controlling several super powered beings, and commanding an army of Sentinels to secure the city, Onslaught seemed unstoppable.
Watching the unfolding drama with Uatu, the celestial Watcher assigned to Earth, Apocalypse attempted to comprehend the nature of this new player and divine his origins. Together, Uatu and Apocalypse soon realized the truth behind Onslaught. Hardly a newcomer to the world of mutants, Onslaught was indeed none other than Professor Charles Xavier, driven mad by the thoughts and ambitions from Magneto. Having decided some months previously that Magneto needed to be stopped, Xavier had absorbed the psyche of Magneto, leaving him a blank slate. Over the intervening months, Xavier's personality had grown darker and had begun to resemble that of the Master of Magnetism himself. Finally driven to the point of insanity, Xavier surreptitiously had sought out sources of power. From his stepbrother Cain Marko, aka the unstoppable Juggernaut, Xavier had wrested the source of his powers, the mystical gem of Cytorrak. Adding to this, the tapped potential of Franklin Richards, son to the first family of the Fantastic Four, made Onslaught a nigh unstoppable force.
Taking one of his rare moments of interference in the world of man, Uatu suggested to Apocalypse a coarse of action, an alliance with the one mutant who hated him most, Cable. After finding the grown Nathan Summers and engaging in a brief battle, Apocalypse managed to convince him of the wisdom of the short alliance.
Onslaught, Apocalypse surmised, would be most vulnerable through the astral plane, one of the sources of his powers. Cable would lead Apocalypse through the astral plane into Onslaught's citadel, where Apocalypse would remove the captive Franklin Richards, greatly weakening the mad mutant. The plan succeeded, but was interrupted by Susan Richards, Franklin's mother, who had invisibly accompanied the pair. Having suspected Apocalypse's motive in wanting to actually kill her son, she found her fears warranted and prevented her son's murder. Unfortunately, the reprieve in battle gave Onslaught the time to escape, prolonging the conflict.
Despite the failure of Apocalypse's plan, Onslaught was eventually defeated, though not without cost. The cost came in the form of the heroic sacrifice of the combined forces of the Avengers and the Fantastic Four. Although the two teams would eventually turn out to be very much alive, their apparent deaths at the hands of Xavier's maniacal alter ego impressed Apocalypse. In the ancient mutant's eyes, from the depths of Xavier's weak, peace-dreaming mind, had arisen a being of power and strength. Surely Onslaught had been among the strong! [Onslaught crossover]
Even after the near world-ending conflagration of the Onslaught crisis, Apocalypse did not deter from his goal of testing the strong and winnowing out the weak. Rarely associating himself with non-mutants, Apocalypse chose his next individual of interest to be the being who had always proclaimed himself to be the 
“strongest one there is,” the Incredible Hulk. Teleporting the green goliath into one of his hidden fortresses, Apocalypse brainwashed the Hulk and transformed him into one of his Horsemen, War. To test this newest recruit in his mission, Apocalypse set War against the New World Order, a shadow cabinet organization founded by Apocalypse’s servant, Ozymandias. The N.W.O. had been told by Ozymandias that he was trying to betray his master, but in truth, the secret organization was founded to be yet another tool of testing for the ancient mutant.
Oblivious of this, the New World Order set the Juggernaut and the Absorbing Man against War/Hulk and were both easily defeated. Apocalypse reveled in this victory as it was a success of not only his creation, but by the utilization of more Celestial technology. Apocalypse’s joy no doubt turned to anger when his control of War soon evaporated. After facing his/the Hulk’s best friend Rick Jones and crippling him with a careless blow, the Hulk’s personality emerged from Apocalypse’s programming. Like the first agent Death, Warren Worthington III aka the Angel, the Horseman’s true personality emerged with the apparent death or injury of a close friend.
Despite this apparent setback, the incident was still a victory for Apocalypse as it was a successful testing of newly understood Celestial technology. The test completed and the Hulk having left, Apocalypse ended the only loose end left, the New World Order. Activating the self-destruct mechanism on the sword of War, which they had obtained, Apocalypse annihilated their headquarters, ending their attempt at global puppetry. [Incredible Hulk (2nd series) #455-457]