After using Black Hand's powers to seemingly kill both Hal Jordan and Sinestro, the Guardians of the Universe used the power of the First Lantern to create the Third Army from their own flesh and will.
The Third Army possess the power to assimilate with a touch, and can affect all forms of life, ranging from humans to giant spiders. They can survive in space, travel through it unaided and possess enhanced strength, capable of even killing a Guardian of the Universe. The only way to avoid being converted initially is to have a Lantern Ring, (Though they can circumvent this by ripping the corpsman's ring arm off) and their weak point, according to the Red Lantern Corps's trial and error, is their eyes.. According to the Zamarons, they have the potential to destroy all intelligent life in the Universe in months.
Rise of the Third Army
Attacking targets all over the Universe, attempts by the Red Lanterns to destroy them have failed, as have the Green Lantern Corps. Their most recent victims are the Spider Guild and former Guardian Sayd.