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"Invasion!" is the third annual Arrowversecrossover event, with episodes of the television series The Flash, Arrow and Legends of Tomorrow on The CW, and is inspired by the 1989 comic miniseries Invasion!. Events at the end of the Supergirl episode "Medusa" on November 28, 2016 marked the beginning of the crossover, which began on November 29 with The Flash, continued on Arrow on November 30, and concluded on Legends of Tomorrow on December 1. Each of the main three episodes were titled "Invasion!". In the crossover, Barry Allen recruits Kara Danvers / Supergirl from Earth-38 to Earth-1 to help his team, Oliver Queen and his team, and the Legends to defeat the alien race known as the Dominators.
Development of a crossover between the three series and Supergirl began in May 2016, when it was learned that Supergirl would move to The CW from CBS for its second season. By the following September the scripts for each episode were written, and were filmed in October 2016. The premise and episode titles were also announced in September. In "Invasion!", the main cast members of each series appear in (at least) their own series; Melissa Benoist appears as Supergirl in the entire crossover. A number of actors and characters from previous Arrowseasons returned to reprise their roles, since Arrow's episode doubled as its 100th.
The three episodes of the crossover received positive reviews, with critics praising the interaction among the actors in each series and each episode's advancing season storylines as part of the crossover. The Dominators storyline was regarded as its weakest link, with reviewers finding the alien race's motivations unclear. Ratings for the episodes were high, with each breaking records in viewership and the 18–49 rating and contributing to the most-watched week on The CW in six years. A fourth crossover, "Crisis on Earth-X", occurred the following year.
"Crisis on Earth-X" is the fourth annual Arrowverse crossover event, featuring episodes of the live-action television series Supergirl, Arrow, The Flash, and Legends of Tomorrow on The CW. The crossover began on November 27, 2017, with Supergirl and Arrow, and concluded on November 28, with The Flash and Legends of Tomorrow. In "Crisis on Earth-X", Barry Allen and Iris West's friends come to Central City for their wedding, only for the proceedings to be interrupted by interlopers from analogous universe of Earth-X where World War II was won by the Axis Powers.