Here's a rough draft (very!) of the history of the One Universe:
There was a flash, and a boom. And two that were separate became one.
ONE UNIVERSE RPG HISTORY
The One Universe is a melding of the histories of both DC Comics and Marvel Comics. Every aspect of history is merged, every character affected.
1939-1946
In the 1940s, the so-called Golden Age of the Mystery Men. There, legends like the All-Star Squadron and the Justice Society of America fought evil the world over in the age of the World War. Captain America was a founding member of both teams, and Green Lantern, the Emerald Knight of Gotham City, stood as the shining example from his age. With the end of the war came the death of Captain America, a death of such great magnitude that it rocked the fledgling superhero community. Shortly afterwards, both the All-Star Squadron and the JSA disbanded in mourning. With the coming of the Cold War, the mystery men faded into obscurity.
2005-2006
The Superheroes Return & The Invasion of Earth
It began, fittingly enough, in New York City. There, a man gifted with the powers of a spider was seen, a marvel of his time. The appearance of the Spider-Man heralded the opening of the floodgates as dozens of costumed champions and horrible villains emerged. The Superman in Metropolis, the villainous Doom in Latveria, and ever more. The heroes were a disparate group, not acting in a unified manner until the monstrous Hulk appeared in Star City during a gathering of the rich and powerful in the mansion of Olliver Queen. Unknown to many, the superheroes were in attendance, figures such as Tony Stark, Clark Kent, and Reed Richards all there. Superman, Iron Man, the Flash, the Batman, Mr. Fantastic, the Thing, and Black Canary all worked together to put down the Hulk. In the aftermath of the colossal battle, many of the heroes were left questioning their purpose. In the melee, Olliver Queen's mansion had been annhiliated and the performing group The Flying Graysons had been killed, leaving only Dick Grayson behind, who was taken into the care of Bruce Wayne. Shortly following this event, a group of explorers in the Arcitc, working for Lex Luthor and Wilson Fisk's LuthorFisk Inc uncovered a shocking discovery. Frozen in the arctic wastes was the long thought dead Captain America, who was quickly reviatlized and brought up to speed on the new world he found himself in. As the world hailed the return of one of it's greatest champions, ominous portents began appearing.
On Asteroid M, Magneto was sent to another dimension, leaving the mutant nation of Genosha leaderless. In Star City, the Green Arrow fought a shadowy assassin named Kanto. Iron Man's armor, aided and enhanced by a Motherbox of New Genesis, gave Tony Stark warnings of impending doom and knowledge of a nightmare world called Apokolips. Finally, the watcher Uatu warned Reed Richards of the coming of a being known as Darkseid.
A day later, it began. With a boom, an invasion fleet unlike any other assaulted Earth. From Tokyo to Paris, LA to NYC, the ships came. At their head was the dread god Darkseid, immortal ruler of Apokolips and master of the Omega Effect, had come to Earth seeking conquest. Against his nigh-omnipotent might stood the assembled heroes of Earth. Even now the battle rages on in New York City, a great assemblage of heroes standing against Darkseid and his forces.