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 will all be facing a very old threat from the pages of DC Comics: The alien race best known as the Domination. The Dominion as it appears in Who's Who in the DC Universe #1. Art by Chris Sprouse, Colours by Tom McCraw.
First appearing in the pages of Adventure Comics in 1967, the Dominion is a race of alien beings -- individually known as Dominators -- that rose to prominence with the 1988 miniseries Invasion!, which saw the lanky creatures team up with a bunch of alien races to invade Earth and wipe out every metahuman (and then make their own army of them on the sly to rule the universe).
There's not much else we know about the crossover aside from the Dominator's role -- and the fact the production team is going to great lengths to bring their comic book appearances to the small screen -- but presumably it will be taking a leaf out of Invasion!'s book, as each show finds the Dominators trying to pick each hero off one by one. An alien invasion seems about the right level of threat needed to bring together Supergirl, Flash, Arrow and the Legends of Tomorrow, doesn't it?