War of Infinite Worlds

The War of Infinite Worlds (無む 限げん 大だい  世せ  界かい の 戦せん  争そ) was an omniversal apocalyptic conflict that resulted in the destruction of all universes, and the recreation of a single positive matter universe (and an antimatter universe, albeit briefly) at the dawn of time. It was the final war in the eternal conflict between good and evil, which eventually led to the extinction of literally nerly everything in existence.

The war is the primary highlight of the events of the Cross Crisis Series, particularly in Project X Zone 3: War of Infinite Worlds, Super Project Cross Tag Battle: Destiny and Chou Super Robot Wars.

The Beginning
The roots of the final war are traced back at the very beginning of time when God created a single universe so that light and life can flourish in the vast infinite void. Heaven, the realm of angels, was also created. However, Lucifer, the most powerful angel in the realm, led a massive billion-year-long civil war in Heaven and attempted to overthrow God by traveling back to the dawn of creation and become the center of all existence. But the plan failed and as a result, the once-singular universe split into an infinitesimal multitude of parallel dimensions and alternate universes called the Omniverse. Eventually, after near total casualties on both sides, the civil war was won by the Army of Heaven led by Michael and the remaining angels of the rebellion, including Lucifer and his seven generals, were banished to Hell.

As a result of the multiversal split, a universe made entirely of antimatter was created and the primordial void of nothingness that existed before Creation was given a physical form and a new identity: the Anti-God, the progenitor of all evil, darkness and chaos.

From there, Lucifer would become Satan, the Great Dark God of the Nameless Void and the ruler of all evil. For billions of years, he and his hellish empire ruled over the Earth that would be called Earth-777, the "corpse of the once-single universe" and mankind that populated the world, in a seemingly never-ending age of sin, horror and deception, until the sacrifice of Jesus Christ, the Son of God, to free humanity from the clutches of sin. Enraged at his first defeat since his banishment, Satan forged the false religion Islam in attempt to destroy Christianity and drag the world down to chaos once again. While Satan started to once again exert his influence over Earth-777, his aura of evil would spread across quadrillions of universes in many eons to come.

Namco × Capcom
Countless ages later, being from the Antimatter Universe banded together and founded the organization notoriously known as Ouma, for the purpose of spreading chaos to many worlds wherever they may go. To counter this threat, Shinra was founded.

Casualties
Though the Genocide of Heroes is an important factor in the events of the game, the known casualties of this multiversal catastrophe are only revealed through flashbacks, cutscenes or implications.
 * Shulk is implied to have been killed in the Genocide of Heroes. In a cutscene in the beginning of the game, Shulk's damaged Monado lays on the ground surrounded by rubble and flames. Later, it is explained that Shulk's team was ambushed by Metal Face and an army of Mechons, resulting in most of them, including Shulk himself, killed in the process, with Fiora captured to become Nemesis Face again. Dunban barely survived the assault but was severely injured with right arm cut off. After the heroes of Shinra free Fiora, when she reunites with her brother Dunban and realizes what happened to Shulk and the others by seeing his broken Monado, she breaks down crying.
 * Chrom explains that their team was ambushed by Validar and almost all of them were murdered, leaving him, Lucina and Robin as the only survivors. It would be revealed later on that Tiki and Sumia were also survivors.
 * Prior to Project X Zone 3, Ichigo Kurosaki and Rukia Kuchiki failed to save Orihime Inoue, as she was killed by Aizen in the crossfire. Throughout the majority of the game's events, the two are slowly trying to recover from that tragedy and eventually become a couple as a way to move forward together.
 * During the Genocide of Heroes in the Sakura Wars universe, while Ichiro Ogami is leading the Imperial Kagekidan in a massive demonic assault on Tokyo and barely survived, Erica Fontaine's Paris Kagekidan did not share the same fortunate outcome however, as most of its members, including Glycine Bleumer, Lobelia Carlini, Hanabi Kitaoji and Coquelicot, were slaughtered, Erica retreating underground, and the French capital in complete ruins.
 * Coquelicot was worse off, as she was raped by Scheisse von Arschloch, a dimension-travelling warlord and head of Ouma Europe, and afterwards had her throat impaled by Scheisse's sword.
 * Also in a flashback, in New York, the New York Combat Revue successfully repelled their side of the demon invasion, however Diana Caprice, a member of the Revue, got herself killed while Shinjiro Taiga battled Ranmaru. Overpowered by Ranmaru, the demon then threw a scythe at him to finish Shinjiro off, but Diana got in the way and took the hit. Desperately reaching for her mech, she couldn't move anymore due to her burns and injuries, made worse with her slowly-dying condition prior. Shinjiro went to her side with Diana complaining on why Shinjiro made her fight fate when she really couldn't. Crying out for life, she disintegrated.
 * June Lin Milliam was found to be cruelly ambushed by Black Hayato. After being saved by Shinra, she reveals that Hayato's cover to attack Bilstein alongside his team was blown and was attacked, with Saturn Dyer and Gamof killed, Hayato captured and June barely escaped. Asking Captain Commando for help, he alongside the rest of the Commando Team moved to save Hayato. But it was all a ruse to capture the Commandos instead, as June had been brainwashed from the start. But later, she and Hayato are freed from Bilstein's control and joined the heroes to avenge Saturn and Gamof.

Trivia

 * The War of Infinite Worlds draws inspiration mainly from the biblical Apocalypse and the events of the 1985 DC event crossover Crisis on Infinite Earths. However, some allusions to Ragnarök of Norse mythology are made.