Crisis on Earth-X
"Crisis on Earth-X" is the fourth Arrowverse crossover event, featuring episodes of 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 the crossover, Barry Allen and Iris West's friends visit Central City for their wedding, only for the ceremony to be interrupted by interlopers from the parallel universe of Earth-X, where the Axis powers claimed victory in World War II. Development for a crossover of the four series began in December 2016 after the release of the previous crossover, "Invasion! ".