<P> The S.T.A.R. Labs facility where the heroes initially meet was made to look like the Hall of Justice in the Super Friends animated series . Stock footage from a building in Cincinnati that was used as the basis for the Hall of Justice was altered by Encore for the crossover . </P> <P> Colin Donnell and Colton Haynes appear as holograms, reprising their roles as Tommy Merlyn and Roy Harper, at the end of the Arrow episode . They appeared as holograms (not physically) due to other commitments which prevented their being re-shot . Guggenheim said that Zoic Studios had to rotoscope them from older episodes noting "It was hard, obviously because they had to work with preexisting footage . Yes, they had (many) episodes to choose from, but it was a lot harder than that makes it sound ." </P> <P> After beginning in the final scene of the November 28, 2016 episode of Supergirl, the first part on The Flash aired on November 29, followed by part two on Arrow on November 30, and concluding with part three on Legends of Tomorrow) on December 1, all on The CW . </P> <P> Despite "Invasion!" only occurring on three of the four CW series, the event, marketed as "Heroes v Aliens", was called a "four - night crossover event" beginning with the Supergirl episode . In Canada, where The Flash, Arrow and Legends air on CTV Television Network, the crossover was marketed as a three - night event entitled "Heroes United". A faux classified U.S. government video was released, set about 70 years in the past, detailing the Dominators' first attempt to conquer Earth . The crossover's trailer was released on November 23, 2016 . </P>

Flash arrow supergirl legends of tomorrow crossover order
find me the text answering this question