“Crisis on Infinite Earths” was first hinted at in the Arrowverse in the pilot episode of The Flash. In September 2018, three months before the release of “Elseworlds” (the Arrowverse crossover in the 2018–19 television season), Legends of Tomorrow executive producer Phil Klemmer said that the next crossover had “a loose shape”.In early December 2018, a week before the airing of “Elseworlds …

https://en.wikipedia.org/wiki/Crisis_on_Infinite_Earths_%28Arrowverse%29