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. No sane individual from any of the other Earths has willingly traveled to this Earth.

History
According to Harry Wells, the history of Earth-X followed the same as that of Earth One, with one minor difference: during World War II, the Nazis emerged victorious after nuking New York, London, Paris, and Moscow, forcing the complete surrender of the US, the UK and 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 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/Black Arrow and his wife Kara/Overgirl.

In 2017, with Overgirl's heart 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 One in pursuit of the Earth Thirty-Eight 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 One 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 Black 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 of Tomorrow as Prof. 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.

Residents

 * Felicity Smoak's Earth-X doppelgänger
 * Hitler (deceased)
 * Kara/Overgirl (deceased)
 * James Olsen's Earth-X doppelgänger  (deceased)
 * Kara's adoptive sister (deceased)
 * Tommy Merlyn/Prometheus  (deceased)
 * Oliver/Black Arrow (deceased)
 * Mick Rory (deceased)
 * Sara Lance's Earth-X doppelgänger (deceased)
 * Winn Schott
 * John/Black Condor
 * Phantom Lady
 * Red Tornado (destroyed)
 * "Metallo" (deceased)
 * "Vibe" (status unknown)
 * "The Ray" (deceased)
 * Darrel/Dollman  (deceased)
 * Leo Snart
 * Unidentified Sturmbannführer
 * Blitzkrieg

From Earth One

 * Barry Allen/The Flash
 * 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 Thirty-Eight

 * Alex Danvers

Trivia

 * Harry Wells has stated no sane person would willingly go to Earth-X.
 * Reverse-Flash is quite obviously pointed out as insane, as he willingly came here and allied himself with Fuhrer Black Arrow and Overgirl.
 * The heroes of Earth One and Earth Thirty-Eight were sent here to be executed, while Fuhrer Oliver, Overgirl and Reverse-Flash conquered Earth One.
 * Mick calls this Earth "Planet Nazi".
 * According to Earth-X Tommy, life here has no choices; it's all about following orders.
 * At least two classifications are given to people in the camp 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 One instead of hitting her homeworld Earth Thirty-Eight.
 * Though it is possible that since he is from Earth One and possibly the future, Reverse-Flash knew that Barry and Iris were getting married and knew that Supergirl was going to attend it.

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 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).