Pariah, a brilliant scientist who discovered a weather-control system and "conquered all disease" from his Earth, discovered the existence of a Multiverse and an antimatter universe. He wanted to learn the secrets and origins of the Earth and would do anything to find them out. He was warned against doing so by his world's scientific council, but claimed that knowledge was more important than his world's legends. He also neglected his wife and children in pursuit of his ambition, something which now haunts him many years later.
He built an antimatter chamber to see how Earth was formed, and watched the beginning of time play before his eyes. But by stepping into the antimatter chamber, he set off a chain reaction which destroyed his Universe. He learned that positive matter and antimatter could not live together. And as it happened, the world's most brilliant scientist destroyed all life except himself because he was in the anti-matter chamber. The explosion reverberated throughout the antimatter universe and freed the Anti-Monitor. When his positive matter universe was destroyed, the antimatter universe expanded to fill the void. The Anti-Monitor realized that when positive matter universes die, he gains more power.
The Monitor found Pariah and used him to beat the Anti-Monitor. Pariah could somehow sense destruction, and being immortalized, he was pulled to wherever the Anti-Monitor struck, showing the Monitor where his brother the Anti-Monitor was. Pariah was than forced to watch as countless Earths from multiple universes were destroyed. On one in particular, Earth-6, Pariah forcefully rescued Lady Quark from the destruction of her universe, leaving her the sole survivor.
Of course as it turns out, it wasn't Pariah's fault the Anti-Monitor broke free. With the aid of countless superheroes and villains, Pariah was witness to the success of Alexander Luthor's plan to save the Earth from the Anti-Monitor's battle.
Over the next few years, Pariah travelled the Earth with Harbinger and Lady Quark, exploring and cataloguing the world's new superheroes. His power of flight seemed limited, but he retained his immortality and ability to sense oncoming disasters (this ability seemed particularly severe during War of the Gods). He also seemed to have developed a kind of fixation with Lady Quark.
Ironically, Pariah was eventually killed by Alexander Luthor during the events of Villains United. However, death would prove to be temporary as Pariah was raised by a black power ring and became a Black Lantern. How his resurrection as a Black Lantern may effect his eventual resurrection as a witness to the end of the universe remains to be seen.
In Other Media
A different incarnation of the character appears in the show, portrayed by Tom Cavanagh. This version is named Harrison "Nash" Wells. In his debut appearance, he states that he is searching for eternium.