The CW's superheroes will face the Dominators in four-show crossover

After battling the villainous Vandal Savage in last season's three-show CW crossover event, the heroes of Arrow, The Flash and DC's Legends of Tomorrow will face an even more formidable foe in this year's four-night event, with network newcomer Supergirl joining the fight. 
Mashable has learned that DC Comics' Dominators will invade the shared TV universe referred to as the "Arrowverse" for this year's crossover, with the story arc spanning four consecutive nights. A premiere date for the c