<P> In September 2017, it was revealed that the title of the crossover would be "Crisis on Earth - X" and that Ray Terrill (The Ray) would make his live - action debut in the Arrowverse, ahead of appearing in the animated web series, Freedom Fighters: The Ray, along with other characters and concepts from that series . Russell Tovey also voices the character in Freedom Fighters . </P> <P> In June 2017, executive producer Marc Guggenheim noted that it would be hard to top "Invasion!" 's threat of aliens, so this crossover would aim to "increase the emotional stakes and the emotional payoffs". The following month, Mericle added that the crossover would be "very much rooted in the DCU ." At the Television Critics Association press tour in August 2017, Pedowitz said that the crossover would involve romance, with Berlanti adding, "our way of making the show bigger this year was to go even more personal, so it's a big life event for a few different people on the show . There are many life events that happen ." In September 2017, in a statement revealing the crossover's title, Guggenheim and Kreisberg said that the crossover was conceived "to be evocative of the annual Justice League / Justice Society (comic book series) crossovers we grew up with and looked forward to as kids ." The story of the crossover was conceived by Kreisberg and Guggenheim . The teleplay for Supergirl's episode was written by showrunners Robert Rovner and Jessica Queller, Arrow's by Mericle and Ben Sokolowski, The Flash's by showrunner Todd Helbing, and Legends of Tomorrow's by showrunner Phil Klemmer and Keto Shimizu . </P> <P> Filming of the four episodes began on September 22, 2017 . Supergirl's episode was directed by Larry Teng, Arrow's by James Bamford, The Flash's by Dermott Downs, and Legend of Tomorrow's by Gregory Smith . </P> <P> Blake Neely, primary composer of all four series, composed the two - and - a-half - hour score for the crossover in eight days at The Bridge Recording Studio in Glendale, California . On December 10, 2017, Neely revealed that a soundtrack for the crossover was close to being released . </P>

How many episodes are in crisis on earth x