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 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 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, and is helmed by the 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.

Residents

 * Hitler (deceased)
 * Kara (deceased)
 * Tommy Merlyn/Prometheus  (deceased)
 * Oliver (deceased)
 * James Olsen  (deceased)
 * General Schott
 * Leo Snart
 * Felicity Smoak's Earth-X doppelgänger
 * Unidentified Sturmbannführer

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

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).