Crisis on Earth-X

Crisis on Earth-X is the name of the upcoming 4 part crossover event between Supergirl, Arrow, The Flash and DC's Legends of Tomorrow. The crossover will air over two nights from November 27th to November 28th, 2017. The episodes that are involved in the crossover are:
 * (Supergirl)
 * (Arrow)
 * (The Flash)
 * (DC's Legends of Tomorrow)

Synopsis
Barry and Iris’s wedding brings the gang together, but things go awry when villains from Earth-X attack the ceremony. All of the superheroes band together with help from their super friends like Citizen Cold, The Ray, Felicity Smoak, Iris West and Alex Danvers to take on their most formidable villains yet. Earth’s mightiest heroes – Green Arrow, Supergirl, The Flash and White Canary – lead their teams into battle to save the world.

Trivia

 * Earth-X is a world where Germany won World War II and Green Arrow, The Flash and Supergirl are evil. It was first referenced in the Supergirl episode.
 * Earth-X is the main setting for upcoming series Freedom Fighters: The Ray.
 * The Ray will make an appearance and his live-action debut in the crossover portrayed by Russell Tovey.
 * A version of Leonard Snart known as Citizen Cold will make an appearance in some capacity.
 * Supergirl will have a more prominent role in this crossover than in the previous crossover.
 * Prometheus from Earth-X will appear in the crossover. However, it will not be the Earth-X version of Adrian Chase.
 * The Flash from Earth-X that is set to appear in the crossover will be portrayed by Tom Cavanagh and will wear a Reverse-Flash styled suit. The character will be a version of Thawne and will be called Dark Flash.
 * It is confirmed in the Crisis of Earth X Sizzle Trailer by Supergirl that there are 52 Earths in the Multiverse. Harry says that Earth-X is the 53rd Earth, outside of the known Multiverse, hence its designation not having a number.