Here is a background I am considering developing for science fiction 2d6 System. To me, this looks like a lot of fun.
Humanity came under alien attack. A massive ship scooped up an entire city, and vanished into the depths of space. Months later, it was back, taking another city. Thirteen cities were taken, all told. After three years of sporadic assaults, Earth’s combined resources brought the surprised ship down.
As humanity struggled to decipher the alien technology and build a space-worthy vessel, a mysterious armada arrived and virus-bombed the planet, then cleansed it with fire. The fifteen million humans that had been taken from the earth were the sole survivors of the human race.
The Yurith, a race of weapons developers, studied humanity from afar for decades. They chose humans as a weapons program, noting their resilience and knack for destruction and imbalance. The Yurith seeded several enemy worlds with humanity, and sold them to several factions, knowing they could sit back and watch as humans did what humans do.
They sold Topeka, Kansas, to the Othuur. Two years later, the humans had adapted Othuur technology, rebelled against their alien masters, and taken control of the planet’s moons. They launched their first expedition into the solar system.
The Hasath Primacy, law-givers of the interstellar void, had enough. They purged Othuur’s colony that had been overrun, and brought charges against the Yurith. In exchange for their survival, the Yurith identified the isolated world that provided the human menace, and the Hasath cleansed it.
Too little, too late. Humanity was among the stars, breeding and rebelling and assimilating. After five years in space interacting with aliens, they had developed uneven levels of education and skill with new technologies, based on the contexts where they had been left. Still, humans can weaponize anything. Seven years into the purge, they retaliated on a massive scale.
Working with Yurith criminals, the people of Mexico City sent a joint strike team that successfully boarded the startled Hasath Primacy flagship and took control of its tesser system, jumping the flagship (and the slave-linked fleet) into a nearby sun. Humanity was victorious in halting the genocidal purge, because with the Hasath Primacy greatly weakened, interstellar war broke out.
Years turned to decades, then centuries, as war burned across the void. Humanity was used in a variety of ways by the aliens who had been exposed to them, some more satisfactory than others. The warriors of Urth (as humans came to be known) were famous for their cunning, deceit, viciousness, grudges, adaptability, and resilience. In twenty short years, they could turn around a new generation of combat-ready soldiers. They had a gift for using technology others had painstakingly developed, and for coming up with creative ways to adapt anything to cause ruthless destruction.
Meanwhile, the city of San Antonio had been emptied into a space arcology as a zoological exhibit for the peaceful Urono. Humans overran their defenses, upgraded the arcology to be a heavily armed space station, and engaged in a quest to reunite the scattered people of Urth. The president dreamed of replacing the Hasath Primacy and bringing law to the chaotic void once again.
Meanwhile, the Hasath Primacy was vastly weakened, their homeworld destroyed by long-time enemies. The survivors decided humanity was to blame for their fall from power and the loss of their homeworld, and that it was time to erase the menace of Urth from the stars forever.
Neither the Alamo (as the San Antonio arcology warship was named) nor the Hasath Primacy had an easy task. Humanity had been spread across the stars. In some places they no longer believed Urth was more than a legend, they had lived in isolation for centuries. In other places they were used by aliens as fuel, batteries, and food. In other places they had become mercenary companies in ships outfitted with dozens of alien technologies. Still, the call of an endangered species to gather is difficult to resist.