Crisis on Earth-X, Part 2

"Crisis on Earth-X, Part 2" is the eighth episode of the sixth season of Arrow, and the one-hundred-twenty-third episode overall. The episode is the second of a four-part crossover with the eighth episode of the third season of Supergirl, eighth episode of the fourth season of The Flash and eighth episode of the third season of 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

 * Felicity referred to Oliver of Earth-X as "Oliver in the High Castle", a reference to the television series and novel, Man in the High Castle, which also depicts a world where the Nazis won World War II. Similarly, the television adaptation explored alternate dimensions in its second season.
 * This is the first Arrow episode to air on Monday.
 * This is the first episode of Arrow in which John Diggle does not appear. This makes Oliver the only character to appear in every episode to date.
 * When Barry believes that the Nazis are visitors from another Earth, Jefferson asks "there are more than one?" This is a discrepancy. In last year's crossover, Barry introduces Kara from another Earth. Jefferson would have known about the multiverse since last year's crossover.
 * This is Metallo's first appearance after his death in Luthors.