The Earth-15 dimension was the so called the "Perfect Universe", but later was know as the "Forbidden Universe". Recently has returned to life as the Blackstar ruled universe.
In the history of this universe, several of the world heroes were succeeded by their proteges, (Batman by Jason Todd, Wonder Woman by Donna Troy, Green Lantern by Kyle Rayner). Meanwhile in other cases, the heroes were a different person (as Dru-Zod being Superman and Atom being Jessica Palmer). Other positive elements were than villians like Lex Luthor were honest persons and evils like Joker were dead.
Destroyed during a rampage of Superboy Prime, millions of lives were lost. The dimension still existed but the delicate structure of spacetime was irreparable damaged, causing paradoxes as the existence of a future from where the being know as Volthoom comes.
Only one fragment of this universe survived: the sole relic of Earth-15 itself was the Cosmic Grail, which would survive the events of Flashpoint. It said is hidden in one of the many worlds of the multiverse.
The dead heroes of earth 15 still haunts this universe where the Golden Lantern guards the relic and watches for any sign of the uncoming crisis than could fall upon the multiverse.
Green Lantern: Blackstars
Earth 15 won a new chance of life in an unexpected way: when the Green Lantern from earth 0 Hal Jordan was used to create a world under the control of Controller Mu and the Blackstars, using the Miracle Machine, it was not earth 0 the universe recreated for give form such reality, but it was earth 15. Using that dead reality as a blank slate, Countess Belzebeth and Controller Mu were given an universe where they were the ones who ruled the cosmos. After the dead of Mu, caused by the schemes of Belzebeth, and the annexion of earth to the Blackstars dominion, Jordan leaded a mutiny against the Countess with the help of several other Blackstars, who were suspicious of thier new leader. However Belzebeth discovered the treason and that derived in a clash between the followers of Jordan and the followers of Belzebeth.
Jordan eventually revealed the true nature of this reality to Belzebeth, explainig how he changed earth 15 by using the Miracle Machine, by giving to Belzebeth and Mu the world they wanted, Jordan then used the Miracle Machine one last time to escape back to earth 0 and to communicate the treason of the Countess to the other Blackstars, making her the most wanted in the Blackstars list of public enemies. The world than Belzebeth wanted so much was now her prision. And maybe, her tomb.