The Omega is the hive mind for the Mimics and the true main antagonist in Edge of Tomorrow.
The Omega is the being that sent the Mimics to Earth, and therefore, it is responsible for the events of the film. Since it is the hive mind for the Mimics, they are telepathically connected to it; for that reason, if it is destroyed, the Mimics cease to function. It also has the ability to reset the day of the battle where the film takes place so that the Mimics can learn and adapt to battle strategies. Coincidentally, Major William Cage (the protagonist) acquired this ability after killing an Alpha Mimic, though he didn't realize it at the time. Sergeant Rita Vrataski (the deuteragonist) had similarly gained this ability, but eventually lost after receiving a blood transfusion. Eventually, Cage and Vrataski discover that the Omega is hiding in France at the Louvre, so they head there. Cage destroys the Omega with a grenade belt, ending the Mimics for good.