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 (known locally to it's ruling government as The Fatherland) is the name given to an unnamed Earth where the Nazi organization won World War II. 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-1 except that 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 SS set up outposts throughout Europe and the Americas from which to establish complete and total domination. The Hitler of Earth-X reigned over his worldwide Reich for another forty-nine years until his death in 1994. Whomever succeeded him and reigns in the present day is currently unknown but by all appearances is no different from their predecessor. Overall order is maintained by a small cadre of evil enforcers that includes Tommy Merlyn/Prometheus, Eobard Thawne/ Reverse Flash, and Metallo, Typhuss James Halliwell/Dark Red Arrow and is helmed by the Fuhrer Oliver Queen and his wife Kara Zor-El. It is unknown if any other costumed villains serve the Third Reich on this Earth or not.

In 2390, with Overgirl's heart on the brink of failure from solar radiation poisoning, the Fuhrer/The Dark Arrow/Oliver Queen resolved to track down another version of Overgirl to steal her heart and give it to his wife to heal her. The forces of Earth-X invaded Earth One 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 of Tomorrow. The invasion was all for naught as, despite their best efforts, careful planning and preliminary successes, the Earth-X villains were outmatched by their Earth One counterparts and in a series of ugly confrontations each met their end; Overgirl going supernova and exploding in space, Prometheus committing suicide, Metallo being blown up, Dark Red Arrow being shot in the heart by Red Arrow and Dark Arrow being shot in the heart by Green Arrow. Only the Reverse-Flash survived but abandoned his Earth-X allies when their defeat became inevitable. With the demise of the Earth-X enforcers, the current status of Earth-X is left unclear.

Residents

 * Winn Schott
 * Leo Snart
 * Felicity Smoak's Earth-X doppelgänger
 * Quentin Lance's Earth-X doppelgänger
 * Hitler (deceased)
 * Kara/Overgirl (deceased)
 * Tommy Merlyn/Prometheus  (deceased)
 * Oliver/Führer (deceased)
 * Typhuss James Halliwell/Dark Red Arrow (deceased)
 * Unidentified vigilante  (deceased)
 * Mick Rory (deceased, mentioned only)
 * Sara Lance's Earth-X doppelgänger (deceased)
 * Kara's adoptive sister (deceased)

From Earth One

 * Barry Allen/The Flash
 * Jefferson Jackson/Firestorm
 * Sara Lance/White Canary
 * Oliver Queen/Green Arrow
 * Typhuss James Kira/Red Arrow
 * Martin Stein/Firestorm (deceased)
 * Raymond Terrill/The Ray
 * Eobard Thawne/Reverse-Flash

From Earth Thirty-Eight

 * Alex Danvers

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).