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 its ruling government as the Fatherland) is the name given to an 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 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, and Metallo, and was helmed by the Führer Oliver and his wife Overgirl.

In 2017, with Overgirl's heart on the brink of failure from solar radiation poisoning, the Führer 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 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, 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 and Oliver 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. With the demise of the Earth-X enforcers, the current state of Earth-X was left unclear.

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 (deceased)
 * Mick Rory (deceased)
 * Sara Lance's Earth-X doppelgänger (deceased)
 * Kara's adoptive sister (deceased)
 * Winn Schott
 * Cisco's Earth-X doppelgänger (status unknown)
 * "The Ray" (deceased)
 * Leo Snart (currently on Earth One)
 * Unidentified Sturmbannführer

From Earth One

 * Barry Allen/The Flash (back on Earth One)
 * Jefferson Jackson/Firestorm (back on Earth One)
 * Sara Lance/White Canary (back on Earth One)
 * Oliver Queen/Green Arrow (back on Earth One)
 * Martin Stein/Firestorm (back on Earth One; deceased)
 * Ray Terrill/The Ray
 * Eobard Thawne/Reverse-Flash (presumably back on Earth One)

From Earth Thirty-Eight

 * Alex Danvers (back on Earth Thirty-Eight)

Season 1

 * Episode 1
 * Episode 2
 * Episode 3
 * Episode 4
 * Episode 5
 * Episode 6

Trivia

 * This is first Earth in the Arrow multiverse that has doppelgangers of people from both Earth One and Earth Thirty-Eight.
 * It's possible that this is the only Earth with a Supergirl doppelganger, as Overgirl targeted the main version of Supergirl to steal a new heart from her.
 * 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 Oliver and Overgirl.
 * The heroes of Earth One and Earth Thirty Eight only came here to rescue their friends and destroy the portal to other Earths to prevent Earth X from invading other universes in the multiverse.
 * Mick calls this Earth "Planet Nazi".
 * This is the first Earth that has gotten a CW Seed series centered on it.
 * Earth-X developed a Meritocracy.
 * It could be assumed to be some type of perverted Nazistic version of a Meritocracy government.

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).