Welcome To the Ultraviolet Lantern Corps
Amazingly enough, the greatest threat to the universe is not Sinestro and the conversion of John Stewart. There is actually an entire Ultraviolet Corps out there under the control of Umbrax, hidden away within the sentient black sun's phantom galaxy. The cosmic entity has taken control of entire planets, leaving an army of villainous Lanterns waiting for a leader to free them. When the Legion of Doom successfully unlocks the Still Force in Justice League #4, it allows the membrane dividing the universe from Umbrax's pocket dimension to weaken.
Sinestro is able to use the power of the Invisible Spectrum to reach out and make a call to everyone on Earth. He preys on the human race's base instincts and tendency to embrace chaos and hatred, recruiting the citizens of Earth into the Ultraviolet Corps. At this moment, Umbrax and his army come over into the universe, and Cyborg takes a moment to reflect that "Sinestro now leads the largest corps in eternity. All the other corps put together aren't even close."
You might be under the impression that the Ultraviolet Corps is just another color of ring wearers in a universe filled with them, but you might want to stop and re-assess. These Lanterns are powered by an entirely separate power source known as the Invisible Spectrum. After spending eons locked away from the rest of the universe, it has revealed itself to be even more powerful than any group of Lanterns before it. There was a reason it was considered too dangerous to control, and now the Invisible Spectrum has returned with a vengeance. If the Justice League is going defeat this threat, it will have to overcome the biggest corps that has ever existed.