Kryptonian Hegemony

The Kryptonian Hegemony exploded out of nowhere ca. 50,000 BCE and became an overnight threat to much of the galaxy's civilized life.

Kryptonians were a single-planet population for millennia, dwelling on a planet with a thick atmosphere with dense heavy metals suspended between air layers. The galaxy was unaware of them as anything except a sub- Type I civilization. When Kryptonians first launched themselves into space, they were outside of their atmosphere barriers for the first time, and exposure to solar radiation supercharged their bodies with unparalleled power.

In less than a century they were spreading like a wildfire. They seemed invulnerable. Little seemed to do them harm, or even slow them down. The Hegemony consolidated their scattered operations into a military operation and directed more aggressive campaigns. Even the least of Kryptonians possessed vast strength, invulnerability, and independent flight. When they blew away the heavy metals obstructing their primary sun, every Kryptonian became empowered as never before. With their increased perception came increased intelligence; Kryptonians realized that glittering sunstones worshipped by primitive tribes were natural conductors of the same cosmic radiation that empowered Kryptonians. These sunstones were naturally-occurring versions of the same crystals that served as the backbone of advanced technology across the galaxy.

Vast coalitions of spacefaring races tried to resist the Kryptonians, particularly Oa. The Hegemony destroyed the Manhunter Robots in legion, a single Kryptonian a match for dozens of those potent machines. All was to no avail, and it seemed likely that Kryptonians would conquer the whole of the known universe if not stopped.

Oa launched a multi-pronged approach. They created the Green Lantern Corps to be more flexible and adaptable than their robotic predecessors, and approached the Elder Gods around Earth to join forces and raise the Demiurge into true awareness. In return, Oa demanded a race of magical warriors bring their fury to bear on Krypton-- the Asgardians answered that call.

Last, Oa attached Kryptonians with a slow-working genetic plague. It spread slowly and undetected for years. It was most of a decade before the Kryptonians realized that they had been rendered sterile. Their already low birth rate dropped to a thousandth of a percent. Combined with attrition from the war, the Hegemony collapsed in the space of a few decades.