Migrants of Multiple Earths Thesis: Part 8 Crisis Conundrem
During the height of the Crisis on Infinite Earths, Lex Luthor and Brainiac assembled a large number of supervillains from the five surviving Earths in order to conquer three of them and ultimately subdue all of them. As a result, the Dastardly Duo sent legions of these costumed crooks to accomplish this task, while the superheroes were divided up in teams to tackle this treat and protect these worlds that were under represented by champions like Earths One and Two were. As an unintended side effect of this, a number of these individuals moved permanently to new worlds in universes they were not born in and seemingly remained there through the end of Crisis conflict.
Among the armies that arrive on Earth-Four were Black Manta, Black Spider, the new Cheetah, Evil Star, Kobra, and Lady Lumar who originated on Earth-One, and Dragon King, Gambler and Solomon Grundy of Earth-Two. This was the last documented sighting of each of these individuals, hence after their collective defeat they remained there. But they were not alone.
Aquagirl aka Tula of Earth-One... a native the Atlantean city of Poseidonis... was among the contingent that traveled to Earth-Four to protect it, and was asphyxiated by Chemo. Such an eventuality would not transpire with her Earth-Two twin, Princess Lanya who lived out her life in the underwater city of Venturia in that Atlantis.
The next world to be assaulted by villains and assisted by heroes was Earth-S. Among the former, were Earth-One's Clock King, Eclipso, Queen Bee and Starro alongside Earth-Two's Cheetah. Those who remained with these miscreants on Captain Marvel's home world were the six Metal Men: Gold, Lead, Iron, Copper, Mercury and Platinum.
The last of the terra firma triad, Earth-X, was itself home to a large number of migrants from Earth-Two decades earlier. This included Doll Man, Firebrand, Human Bomb, Phantom Lady, as well as others who have since left that world behind in Jester, Manhunter with his dog Thor, Plastic Man, Quicksilver and the Spider. That world only had two from this conflict who just arrived remain on Earth-X... Fastball of Earth-One and the Mist of Earth-Two.
Concurrent with these events, the she-robot Mekanique of Earth-Two ended her temporary stay on Earth-One under Commander Steel's employee, disappearing from before the combined might of the Justice Society and Justice League. Not reappearing on either of their worlds, there are limited locals she could've ended up as the Crisis was under way... Earths Four, S and X being prime candidates for where she now was.
Factoring in additional heroes from Earth-One's Justice League of Black Canary (herself originally of Earth-Two) and Steel (who's grandfather originated on Earth-Two) making the journey to Earth-S... and who didn't return to Earth-One prior to the final conflict...the scales were balanced. An equal number of persons from both Earths One and Two left these behind for new universes. But there were other notable departures during this Multiverse scale conflict.
Prince Gavyn aka Starman perished while protecting his planet-less people due to an anti-matter wave. Gavyn's disappearance from Earth-One's universe was corresponded with the appearance in the universe to another alien in the Bottled City of Kandor, now residing in Earth-One Superman's Fortress of Solitude. This was in the form of Jur-LL, the Earth-Three counterpart of Superman's late father Jor-El, who was a criminal mastermind who battled Nightwing and Flamebird before himself perishing years before the Crisis.
Another former occupant of Earth-Two who for a brief period made his home on Earth-One was the Immortal Man. In his most current form, he used his abilities to protect others when he couldn't save his own life. Since he wasn't a native there in the first place. Nighthawk of the 18th century Earth-One along with a pair of lasses also perished due to an anti-matter wave along with his horse, while the four Losers during Earth-Two's World War II also perished from "shadow demons", enhanced soldiers from the Anti-Matter Universe.
Instances of Crisis-level events precipitating the transfer of inhabitants from one universe to another had occurred in the years proceeding these monumental event. For example, the Earth-One Champion family... all four of them... left for Earth-Two after indirect interference with one of the Monitors responsible for the Crisis.. Another quartet, the Newsboy Legion, had come from Earth-Two and moved to the Champion's former world during a crisis of their own.
Then there were the instances of "Crisis on Earth-One" or "Crisis on Earth-Two", etc. Such as when the Justice Society met Earth-One's Legion of Super-Heroes and the Justice League met Earth-Two's Law's Legionnaires. Or when allies of the Justice League who were (at that time) non-members such as Elongated Man, Metamorpho and Zatana joined them on a case to save Earth-Two, while villains such as the Shade, Thinker and the Fiddler tormented Earth-One's Flash, Atom and Teen Titans. While not permanent migrants, these individual had prominent and often significant effects on these alternate earths thanks to their influence on these occasions.