Tomorrow night kicks off the 4-series crossover event for the CW and the bad guys in it are the Dominators. The event follows the comic mini-series Invasion that DC did in 1988 / 1989. So before that starts, lets do a quick primer by going over the comic book history of the Dominators.
Collectively known as the Dominion, and are a highly technologically advanced race from the outer cosmos. They exist in a caste based hierarchical society where your rank is determined by the size of the red mark on your forehead.
Interestingly, their first appearance chronologically is not their first appearance in print. They were created by Jim Shooter for Adventure Comics #361 as opponents for Superboy and the Legion of Superheroes in the 30th century. Originally depicted as blue skinned, the Dominion waged a long war against the United Planets. The LoSH were tasked to escort a group of Dominators to secret peace talks only to be attacked by a group led by another Dominator who opposed peace. The war would expand over time but eventually peace was established.
Their next appearance is when they showed up in the 20th century as part of an alien alliance that attacked Earth. The reason for the attack was a fear of the genetic potential of human-kind. The evidence being the number of super-powered humans and the existence of a meta-gene. The invasion doesn't go well, so a single Dominator decides to create a gene-bomb that goes off in the atmosphere affecting many individuals that possess the metagene, causing them to lose control of their powers and eventually fall into a coma and die. This was against the purpose of the mission, which was to find a way to harness the powered humans, so that Dominator was sentenced to death.
A small group of heroes unaffected by the bomb, lead by Martian Manhunter, found the Dominator prior to his execution and stole information from him mind that would allow them to reverse the effects of the gene-bomb. An ironic side-effect of the gene-bomb is that some of the humans effected by it had previously been non-powered, their meta-gene was dormant. But the bomb and the reversal awakened their abilities.
The Dominion would return to haunt the Legion of Super-Heroes over the years, appearing as part of the Terra Mosiac, where they gained control of the Earth's government after a galaxy wide economic collapse. Their purpose was again to experiment on Humans. They caused the Legion to break up at one point, but they eventually lost control of the Earth due to a rebel uprising… and having one of the Linear Men accidentally blow up the moon with nuclear weapons the Dominion had place there.
What appears to be the final fate of the Dominion involved Cosmic Boy ordering Mon-El detonate a bomb that appeared to destroy the Dominion's home world, but in fact the bomb pushed them into the Phantom Zone where they are trapped. A fitting ending as Mon-El is a Daxamite and the Dominion had made a play to take over Daxam in hopes of building a super-powered army. Glorith destroyed Daxam to prevent this from happening.
But then again, DC keeps rebooting not only their main universe but the Legion often enough that it's hard to say exactly if the Dominion is gone or not.