Disclaimer: I own no published intellectual properties mentioned in this fic.
All it took was a single Spark to throw the entire multiverse into chaos. That Spark ignited other Sparks in other realities, which cascaded into seemingly infinite Sparks, at least one space-time moment in each universe.
Of the 435,500,000,000,000,042 inhabited universes in the Multiverse, only 31,512,896,371 survived the initial erupting Sparks. Those universes that did not survive were erased from existence immediately, people, places, things, everything gone.
Of the 31,512,896,371 surviving universes, 24 had to be sacrificed to save the remaining 31,512,896,347 realities.
The collective of higher powers overseeing the multiverse were not unkind in their selection of which universes survived and which 24 were sacrificed. Those mortals that they could save within the selected ones, they did, after a fashion. Those that they could not save, they ensured their sacrifice was painless and instantaneous, with their souls preserved if the individual soul wished it. Those in charge of the surviving soul's afterlife provided comfort and counseling based on their individual beliefs.
Janus, the god of portals, did his part. As quickly as he could, the god opened as many portals in the 24 universes as possible. Each portal was supposed to lead to Earth 93,569,184, a new and exciting planet that would have human life in the distant future and would allow the survivors to live in peace and prosperity for generations. There was no time to aim, though, so the portals instead led to 93,569,185. That world was inhabited, full of history, violence, intrigue, and other dangers. Janus had messed up big time.
To those that were saved, it was as if they saw a blinding flash of light and were immediately in a different place. Janus, working quickly to mitigate his understandable targeting error, tried to place his favorite acolytes together, hoping they would band together and survive. He spent some of his power, even, to give them some boons to use.
Six seconds after the Sparks destroyed, the lives of thousands were changed, and they would never know how close they were to their utter annihilation, or instantaneous death.
On the new world, three quarters of the survivors die with the first day, due to hostile natives, poisonous flora, and hungry fauna, or being dropped into the middle of the ocean by a misplaced portal, or, in the case of seven unlucky souls, an erupting volcano.
Those that survived, however, forged their way forward into a shiny new world to call their own, as long as they could survive.