LEGION

The Tchk-Tchkii were a race of people that were basically super intelligent ants. They built hives and quickly used up all of their home planets natural resources. Because of this, and because of their simplistic nature, they looked to simply move to a new planet and build there showing no remorse for their wasteful ways. Because of this, the Guardians of the Universe on Oa determined that the best course of action was to keep the Tchk-Tchkii on their planet by placing a giant forcefield around it.
 
Unable to change their ways, or fix their dying planet, the Tchk-Tchkii started to die slowly. In a last ditch effort to save their race they built a machine to contain the life force of their people. This caused them to become a protoplasmic type ooze. They then used a set of yellow power armor that they created to break through the forcefield and seek revenge on the Guardians. Not knowing the location of Oa, the Tchk-Tchkii settled for killing Green Lantern after Green Lantern in an effort to extract the location of this information from the various ring bearers. This brought them into conflict with Abin Sur of sector 2814. Abin was able to fend off Legion, but sustained severe injuries in the process. He was able to survive long enough  to crash land on Earth and pass the mantel of Green Lantern on to Hal Jordan.
 
Legion attacked Jordan once it was able to track him down. Hal eventually, and quite accidentally, led  Legion to Oa. He then started systematically murdering Green Lantern after Green Lantern as he made his way towards the Guardians. He was held back, temporarily when Hal Jordan knocked Legion into the mud of Oa, thus negating his yellow color and allowing him to use his ring against Legion. He shelled Legion of its armor, only to face a protoplasmic blob that was even more powerful. Legion dug into the planet Oa and began siphoning off its latent power using it to combat the lanterns. Feeling completely overwhelmed, the Guardians began evacuation of the planet. Hal Jordan, being unable to accept their order and defeat, flew into the central power battery and used its power to eject Legion from the planet. The protoplasmic goo was then brought back to the Tchk-Tchkii's native planet to start its evolution all over again, hopefully into something less destructive.