Earth-X

"There's a 53rd Earth, and it's called Earth-X. It doesn't have a designation because it's a place so awful, so horrific, no sane person would ever travel there."

- Harry Wells

Among the many universes in the multiverse, Earth-X is the name given to an Earth where the Nazi organization won World War II and created a worldwide New Reich called The Fatherland. Due to its horrific state, it doesn't have a formal designation, since no sane individual from any other Earth has willingly traveled to this Earth. That all changed when visitors from two dimensions aided the local resistance movement and took down the New Reich's leadership and arrested one last member of the totalitarian regime, thus liberating the planet.

History
According to Harry Wells, the history of Earth-X followed the same as that of Earth-1, with one major difference: during World War II, the Nazis emerged victorious after nuking New York, London, Paris, and Moscow, forcing the complete surrender of the US and the UK, while annihilating the USSR altogether. After that, the Nazi forces spread out across the world, conquering everything in their path, including their former allies in Europe and Asia. As they did so, the Schutzstaffel set up outposts throughout Europe and the Americas from which to establish complete and total domination. The Führer Hitler reigned over his worldwide Reich for another forty-nine years until his death from natural causes in 1994. As of 2017, order was maintained by a small cadre of evil enforcers that included Tommy Merlyn/Prometheus, Eobard Thawne/Reverse Flash, Metallo, and was helmed by the Führer Oliver/Dark Arrow and his wife Kara/Overgirl.

In 2017, Overgirl flew to close to the sun so her heart was on the brink of failure from extreme exposure to solar radiation, the Führer resolved to track down another version of Kara to steal her heart and give it to his wife to save her. Through experimentation with trans-dimensional technology, the Earth-X villains discovered the means to travel between the various Earths. The forces of Earth-X subsequently invaded Earth-1 in pursuit of the Earth-38 Kara/Supergirl and recklessly attacked during the wedding of Barry Allen/The Flash and Iris West, but were decisively beaten back by the combined might of Supergirl, Team Flash, Team Arrow and the Legends. The invasion was all for naught as, despite their best efforts, meticulous planning, and preliminary successes, the Earth-X villains were outmatched by their Earth-1 counterparts following a series of ugly confrontations, culminating in a massive street battle in Central City, each met their end; Overgirl going supernova and exploding in space, Prometheus committing suicide, Metallo being completely destroyed and Dark Arrow being shot in the heart by the Green Arrow. Only the Reverse-Flash survived but abandoned his Earth-X allies when their defeat became inevitable. Despite the invasion being a failure, it nonetheless caused massive collateral damage on both Earths and dealt an especially devastating blow to the Legends' team as Professor Martin Stein was killed in action against the Nazi forces, resulting in the loss of the Firestorm Matrix and Jax leaving the team soon after for the foreseeable future.

Months later, it is revealed by Leo Snart that the deaths of the Earth-X leaders turned the tide of the rebellion against the Nazis and that the New Reich has since been dismantled completely, ultimately restoring Earth-X to a peaceful world. The last Nazi loyalist, Siren-X, eluded capture for months, until she was finally apprehended by Leo and Team Flash on Earth-1, and subsequently imprisoned by the new Earth-X authorities.

From Earth-1

 * Barry Allen/The Flash
 * Cisco Ramon/Vibe
 * Jefferson Jackson/Firestorm
 * Sara Lance/White Canary
 * Oliver Queen/Green Arrow
 * Martin Stein/Firestorm (deceased)
 * Ray Terrill/The Ray
 * Eobard Thawne/Reverse-Flash

From Earth-38

 * Alex Danvers

Locations

 * London (destroyed)
 * Moscow (destroyed)
 * New York (destroyed)
 * Paris (destroyed)
 * Star City
 * Freedom Fighters' base of operations
 * Tulsa

Trivia

 * This Earth is nicknamed “Planet Nazi” by Mick Rory and “Nazi Earth” by Cisco Ramon and Ray Terrill.
 * According to Tommy Merlyn, life here has no choices; it's all about following orders.
 * At least two classifications are given to people in the camp that Oliver, Sara, Stein, Jax, Alex and Barry were sent to.
 * Yellow Star of David badges are for Jewish people.
 * Pink triangles badges are for non-heterosexuals.
 * Earth-X developed some form of a meritocracy.
 * It is never explained how the Earth-X villains knew to find Supergirl on Earth-1 instead of hitting her homeworld Earth-38.
 * Though it is possible that since he is from Earth-1 and from the future, Reverse-Flash knew that Barry and Iris were getting married and knew that Supergirl was going to attend the wedding.
 * It's also possible that the Earth-X Nazis knew Supergirl was visiting Earth-1 because, as stated by Overgirl, they'd been watching the heroes on both Earths.

Behind the scenes

 * In DC Comics continuity, the Nazis play the same role on Earth-X as they do in the shared multiverse. However, it is considered a part of the main 52 universes in the multiverse, rather than something outside of it, with its alternate versions in the continuity being known as Earth-10 (though it is still called "Earth-X", but in this case it is because the "X" is the Roman numeral of 10).
 * There is a number of universes considered to be outside the main 52 universe in the comics, most recently the Dark Multiverse: a collection of "minus universes" born out of people's fears, regrets and nightmares, doomed to disintegrate and be superseded by the new "minus Earths". And like on Earth-X, heroic characters from mainstream continuity are twisted villains there (alternate versions of Bruce Wayne/Batman, namely).