First Look At Dominators in Arrow, The Flash, Supergirl & Legends of Tomorrow Crossover Trailer
The Dominators are a mind-controlling alien race that make war with Earth over the Earth's metahumans causing the CW superheroes to battle each other and more (lots of details here).
The storyline is based on the DC Comics Invasion! storyline and kicks off starting with Supergirl on Monday, November 28th.
Check out the Supergirl: Heroes vs Aliens trailer: