The Return of Apocalypse was an event that occurred in 1983 of the revised timeline. It saw the world's first mutant, Apocalypse, awaken from a millennia-long slumber and consequently deciding to destroy the world to remake it in his image.
In the year 3600 BCE, Apocalypse ruled over Egypt with his Four Horsemen at his side. However, some of Apocalypse's followers saw him as a "false god", and plotted his downfall. On the day that Apocalypse prepared to transfer his consciousness into the body of a new mutant, a group of conspirator guards unveiled huge blocks that they used to facilitate the destruction of the pyramid where the transference ritual was taking place. As the pyramid collapsed around them, the guards inside tried to attack a dormant Apocalypse in his new body, but the Four Horsemen quickly disposed of them.
However, three of the Horsemen met gruesome fates from being crushed by the debris of the crumbling pyramid. Before she was killed herself, the last remaining Horsemen used her telekinetic abilities to create a protective barrier around Apocalypse's inert body, allowing him to survive under the ruins of the pyramid for the next millennia. 
In the aftermath of Apocalypse's downfall, the humans began reforming the world, leading to the eventual creation of the U.S. Government and the birth of technology such as nuclear weapons. Additionally, more pyramids were constructed by Egyptian slaves a millennium after the coup.
Reawakening in 1983
In 1973, due to the actions of a time travelling Wolverine, the existence of mutants was exposed to the world at the Paris Peace Accords.  In response, a number of cults have formed under the belief that mutants were a sign of God. One of these cults, Ashir En Sabah Nur, worshiped Apocalypse and their activities caught the attention of the CIA. Agent Moira MacTaggert was assigned to keep track of them.
In 1983, MacTaggert followed Ashir En Sabah Nur to the underground ruins of Apocalypse's pyramid in Cairo. However, she unknowingly allowed sunlight to enter the chamber, which completed the transference ritual and revived Apocalypse from his slumber. As Apocalypse teleported away, his power caused the underground chamber to destabilize and collapse, killing Ashir En Sabah Nur and triggering tremors around the world. MacTaggert was able to escape just in time, and returned to CIA Headquarters with the information she had obtained on Ashir En Sabah Nur. 
Arriving in Cairo, Apocalypse wandered through a busy marketplace, where he witnessed a young Ororo Munroe use her mutant abilities to steal cash from a vendor. The vendors pursued Munroe and cornered her with the intention of cutting one of her hands off for stealing. At that moment, Apocalypse arrived and used his powers to behead several of the vendors, entrapping the last remaining vendor in a wall.
Munroe brought Apocalypse to her place, where he was able to tap into the radio waves from Cairo's satellites to learn about the modern world and the millennia of history that unfolded while he was dormant. Now able to speak English, Apocalypse told Munroe about his plans to "save" the world from the "false gods" that have taken the Earth. His first act in doing so was to enhance Munroe's powers so that she could be one of his Horsemen. 
Garbed in new attire, Munroe brought Apocalypse to the location of Caliban, a mutant who could help them find other mutants powerful enough to join Apocalypse's Horsemen. Caliban refused to help without payment, and aimed a gun at Apocalypse, who proceeded to disintegrate the weapon with just a glance. Caliban's bodyguard, Psylocke, quickly aimed her katana and psionic blade at Apocalypse and Munroe. Sensing the true potential of her power, Apocalypse enhanced Psylocke's abilities and convinced her to join his cause. Enthralled by Apocalypse's promises of power, Psylocke agreed to locate mutants who could join their crusade. 
|Events in the Revised Timeline|
|Coup on Apocalypse (3600 BCE) • Cuban Missile Crisis (Cuban Beach Battle) (1962) • Vietnam War (1955 - 1975) • Saigon Rescue Mission (1973) • Pentagon Prison Break (1973) • Attack on the White House (1973) • Massacre in Poland (1983) • Return of Apocalypse (Nuclear Weapon Incident - Rescue at X-Mansion - Rescue at Alkali Lake - Battle of Cairo) (1983) • D'Bari Invasion (Endeavour Rescue Mission - Red Hook Incident - Train Chase) (1992) • Chase of Ajax (Highway Massacre - Scrapyard Showdown) (2016) • Assault on DMC Convoy (2018) • Battle at the Essex House (2018) • Westchester Incident (2029) • Massacre at Munson Farm (2029) • Chase Through the North Dakota Woods (2029)|