The Green Lantern Corps are some of the DC Universe's most powerful forces, working to ensure peace is spread across the galaxy. But this mission has resulted in them gaining numerous enemies that spread across all of creation. If they all came together, they could be too much for the heroes -- and one time in the Post-Crisis DC Universe, they almost were.
Alan Moore's Pre-Crisis Ragnarok for the Green Lantern Corps was one of the darkest possible endings for the space-faring police force, and it almost came to pass during the course of the "Sinestro Corps War." But what kept it from truly coming to fruition?
In the Pre-Crisis story "Tygers" by Alan Moore, Kevin O'Neill, John Costanza, and Tony Tollin from the Tales of the Green Lantern Corps Annual #2, Abin Sur explores the universe years before his demise. He ends up coming across the Empire of Tears on their world of Ysmault while searching for survivors of a crashed ship. The deathless Empire of Tears reigned in chaos for untold eons until they were defeated and imprisoned on their world by the Guardians of the Universe. Abin specifically finds Qull of the Five Inversions, who agreed to answer any three questions Abin Sur asked. In the process, he earned two dark prophecies from the mystical being. The first teased that Abin Sur would soon die as a Green Lantern and that Hal Jordan would succeed him and become a legend.
The other spoke of the "Final Catastrophe," wherein the enemies of the Green Lantern Corps (including the Weaponers of Quard, Ranx the Sentient City, the Children of the White Lobe, and the Empire of Tears) united and assaulted Oa, wiping out most of the Corps in the process. The last hope of the Corps would fail when Sodam Yat, a Daxamite heralded as the Ultimate Green Lantern, dies at the hand of the Children of the White Lobe as Ranx destroys Mogo. It's a horrifying ending for the Green Lantern Corps, defeated and destroyed, leaving the army that destroyed them to bring chaos back to the universe.
In the Post-Crisis DC Universe, this became known to the Guardians as part of the prophecy of the Blackest Night, a day foretold to mark the end of the Corps. However, the prophecy was shared with the rest of the Guardians, leading them to further fear the prophecy. This also resulted in Sinestro and the Green Lantern Corps, shortly after Abin learned of the prophecy and perished, ensuring that the Empire of Tears were contained in their world instead of being brought to the Sciencells on Oa, where they could have escaped and participated in the coming battle. This meant that when the time of the Sinestro Corps War came, they were not willing or able to fight alongside him against the Green Lanterns.
This quietly turned the tide of the war. Alongside the Weaponers of Qward, Ranx the Sentient City, and the Children of the White Lobe, the Sinestro Corps almost broke the Green Lantern Corps in a massive ambush across the galaxy. The Battle of Mogo ended up being a deciding factor in the conflict, with many Lanterns flying to the defense of the living world -- including the rookie Lantern, Sodam Yat. Without the additional help of the Empire of Tears though, the Green Lanterns were able to hold the line long enough for the Guardians to reverse the rule against killing their enemies -- giving the far more experienced and well-trained Green Lantern Corps an edge over their enemies.
Sodam Yat was able to breach the core of Ranx and destroy him, escaping unscathed and cementing the beginning of his legend as a great Green Lantern. Meanwhile, the Children of the White Lobe were unable to escape into the heart of Mojo and detonate, allowing him to survive as well. It's interesting to consider just how close the Sinestro Corps War came to being the end of the Green Lanterns as a whole. Without Mogo, the Corps wouldn't be able to summon new beings from across the universe to fill the ranks of the Lantern Corps. If the Empire of Tears had been involved, then this alliance of ancient enemies might have been too much for the Green Lanterns.
After the battle was won and Sinestro was captured, he revealed to Hal Jordan he expected this ending -- and now believed that the Corps would grow stronger as a result, allowing them to survive whatever threat could otherwise be the Blackest Night. It also directly led to Attrocitus killing the rest of the Empire of Tears in a rage and using their blood as part of the process to construct the Red Lantern.