Crisis on Earth-X

Crisis on Earth-X is the name of the fourth annual shared multiverse crossover event between, , and. The crossover aired over two nights from November 27th to November 28th, 2017.

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 by Barry as a hypothetical example of how various Earths in the Multiverse may differ from one another while explaining the concept of the Multiverse to Kara, James, and Winn.
 * Ray Terrill/The Ray made his live-action debut in this crossover. His own animated series: premiered on CW Seed on December 8, 2017.
 * Unlike previous crossovers, Crisis on Earth-X received its own special opening title card and did not have "previously on" at the start of each part.
 * The only series regulars who did not appear in the crossover were:
 * - Katie McGrath (Lena Luthor) and Odette Annable (Samantha Arias)
 * - Neil Sandilands (Clifford DeVoe)
 * - Willa Holland (Thea Queen) and Katie Cassidy (Earth Two Laurel Lance)
 * Supergirl reveals that there are 52 Earths in the Multiverse. Harry says that there are actually 53 Earths, but Earth-X is so horrible that no one goes there and therefore it does not have a formal designation. However, it is not indicated how or when Kara and Harry came to possess this information.
 * This is contradictory to Martin Stein's statement in and Harry's statement in  that there are "infinite" Earths and Red Tornado's statement in  of  that there are "thousands" of Earths. Although, it's possible that they simply meant that there are 53 known Earths in the Multiverse.
 * Ralph Dibny, John Diggle, Curtis Holt, Rene Ramirez, Dinah Drake, Ray Palmer, Nate Heywood, Amaya Jiwe and Zari Tomaz were the only heroes not in attendance at Barry and Iris's wedding. That fact was referenced by Ray Palmer in ​​.
 * There are two similarities between this crossover and the Invasion! crossover:
 * The Dominators wanted Barry Allen to surrender himself, while the Earth-X Nazis wanted Kara Danvers to surrender herself.
 * Both the Dominators and the Earth-X Nazis invaded Central City on Earth One.
 * This was the first crossover to not reference Oliver shooting Barry which happened in the first crossover.
 * This crossover introduced Nora; Barry and Iris's daughter. However, since she is from the future she did not reveal her identity to her parents and their family and friends until several months later.
 * This is the first crossover to feature Iris West and Harry wells in all episodes. Previously, they had only appeared in episode of the crossovers.