We already know
that Kara, Barry, Ollie, and the Legends are all teaming up for a one week-long crossover on the CW, and now we know why. They’ll all be facing a very old threat from the pages of DC Comics: The alien race best known as the Domination.
Source: io9 – The CW’s Big DC Superhero Crossover Will Pit the Heroes Against an Alien Invasion