WARNING: The following contains major spoilers for Green Lantern: Blackstars #3, by Grant Morrison, Xermanico, Steve Oliff and Steve Wands, on sale now from DC.
In the final moments of The Green Lantern, by Grant Morrison and Liam Sharp, the villainous Controller Mu used Hal Jordan and his Green Lantern Power Ring to fuel a prototype Miracle Machine, making his deepest wish a reality. Mu wished for a version of the DC Universe where the Green Lantern Corps never existed. This would then put the Blackstars in charge of keeping the peace across the DCU. However, instead of a law enforcement organization, the Blackstars are intergalactic conquers, subjugating the entire universe under their brutal will.
While Hal only appeared to have his memories of the previous iteration of the DCU and his time as a Green Lantern while serving as a prominent member of the Blackstars, the Emerald Knight reveals his long game master plan after the Blackstars defeat the DCU's familiar heroes to conquer Earth. Staging a coup against Belzebeth, Hal's wife from the vampire planet Vorr and leader of the Blackstars after Mu's apparent death, Hal and Belzebeth are the last two standing as they reach the Miracle Machine, defunct without a Green Lantern Power Ring to activate it. As he confronts his wife, Hal reveals the truth about the Blackstar-ruled version of the DCU: It wasn't created by Mu at all.
When Hal was serving as a conduit for the Miracle Machine in the main DCU, he sought an alternate universe where Mu's wish of a reality without the Green Lantern Corps could be fulfilled without rewriting their existing universe. Secretly selecting the dead universe of Earth-15, Mu's dark wish rewrote the abandoned alternate universe, with the villain and his Blackstars none the wiser about Hal's deception. Even in the face of this shocking revelation, Belzebeth taunts Hal that he will be unable to return to his home world until Hal reveals that he had stolen his seemingly drained Green Lantern Power Ring from her after she had usurped Mu and taken the ring as her personal trophy.
Reunited with his Power Ring, Belzebeth is horrified that the ring still holds enough of a charge in this alternate universe to power the Miracle Machine once again. As Hal returns to his own universe, leaving this nightmarish incarnation of the DCU behind for good, Belzebeth is confronted by Controller Mu's armada, with the intergalactic despot revealing he had spread his consciousness across multiple entities and is seeking revenge for Belzebeth seizing control of the Blackstars from him, the vampire villain's past coming back calling in full force.
Throughout the comic book miniseries, it was assumed that Controller Mu had enacted his devious scheme to rewrite the DCU in his image. The truth is, Mu rewrote a DC Universe, just not the DC Universe. And while the wholesale revision of Earth-15 affected Hal's memories, the Green Lantern's true mission was never truly out of sight. The main DCU has been evidently safe all along, with Hal Jordan using a dead universe as part of a desperate ruse. And with Controller Mu and Belzebeth still stuck in this world, the Corps' greatest threat has effectively been isolated and imprisoned elsewhere.