It turns out that many of the threats the DC Universe safe can all be traced back to one particular source. Hal Jordan's earliest days as a Green Lantern revealed a legacy of pain and torment that begins with the Green Lantern Corps' founders, the Guardians of the Universe.
As some of the oldest beings in the DCU, the Guardians have witnessed the growth of the universe over eons. Taking it upon themselves to achieve peace in a lawless world, they tried several notable attempts to establish order, but found their greatest success with the Green Lantern Corps. Having harnessed the Green Light of Will, the most stable light in the Emotional Spectrum, they were able to create an organization that has protected the universe for billions of years. Though they often face criticism over their leadership skills, it's no doubt the Corps has been a success.
But that success has come at terrible cost that can be seen firsthand in the Green Lantern storyline "Secret Origin" by Geoff Johns and Ivan Reis. The future leader of the terrifying Red Lantern Corps, Atrocitus, is on the lose on Earth thanks to Hal Jordan’s predecessor, the Green Lantern Abin Sur, being misled by an ancient prophecy. It’s revealed that Atrocitus is actually one of the few survivors of a brutal massacre committed by the Manhunters, the Guardians’ former peacekeeping force. A programming glitch caused them to wipe out an entire sector, putting Atrocitus on a path of vengeance. Atrocitus fashions a device and seeks to steal a great power prophesized to be within William Hand. However, after Atrocitus is apprehended, Hand makes off with his invention, which would be the same weapon he would use as the supervillain Black Hand.
Black Hand’s weapon helped him fulfill the death-filled Blackest Night prophecy by killing himself and giving the death entity Nekron a connection to the living world. For eons, the Guardians were warned of a War of Light that would end in a wave of death that would spread across the universe until nothing was left. In their attempts to prevent this calamity, the ancient beings made decisions that led to the DC Universe nearly being destroyed.
It all began with the Guardians' creation of the Manhunters, who they thought could effectively patrol entire galaxies. If they had ed for the potential of glitches, Sector 666 wouldn’t have been massacred and the Red Lantern Corps leader Atrocitus would never have sworn vengeance. But the Manhunters' genocide sent Atrocitus on a path to kill William Hand and take his power. Because of that, Hand became a supervillain and his fascination with death led him to becoming the avatar for the Black Lantern Corps. Though the universe prevailed in the face of a zombie apocalypse, everything that happened in the up to it can be traced back to the poor management of the Guardians. In the efforts to protect the DCU, the Green Lanterns' leaders only managed to put it on a path to its near-destruction.
