Half Life: Pandemic

Prologue: Seven Hours

19 YEARS BEFORE OUTBREAK

When the Black Mesa Incident ended, Portal Storms raged across the planet; bringing forth life forms from otherworldy dimensions. These alien life forms included the Xenian Wildlife discovered in Black Mesa and the Cryonian Wildlife, a mysterious alien race that appeared on the planet. Earth governments began setting up protection centers to deal with the hostile aliens that have invaded the planet.

The alien life forms became destructive towards the life forms on Earth, leading to the extinction of other creatures and animals. In the middle of these events, Cryonian Wildlife have migrated into the cold regions of the planet due to being unable to adapt in warm environments. Xenian Wildlife, however, had greatly spread across many parts of the planet, making them much of a larger threat than the Cryonian aliens.

As the Portal Storms continued to spread across the planet, dimensional fluctuations had attracted the attention of the Combine, an immense and powerful inter-dimensional empire.

The Combine launch an all-out assault on Earth, and the United Nations declared war against them. In merely seven hours, the Combine had completely overwhelmed all of Earth's military forces. The entire human race was unable to form any effective defense and resistance against the Combine, leading to the United Nations completely surrendering to the attackers.

Before the events of the Portal Storms completely came to an end, one last alien life form made it's way into Earth. This alien life form, unlike all the others that invaded the planet, appeared to resemble a microscopic organism.

During the Combine's occupation on Earth, Combine cities were built from the remnants of the cities on Earth. This included City 17, where the Citadel descended from the Combine Overworld down to Earth, and Nova Prospekt, a former prison that had been tranformed and refurbished into a facility by the Combine. The Combine are known to have committed extreme human genocide, installed the military system known as the Overwatch, drained the entire planet from it's natural resources and put an end to human reproduction through a worldwide suppression field.

Nineteen years later after the Seven Hour War, the unknown alien micro-organism managed to survive in the Wastelands and remain unnoticed by both the Combine and humanity. Over the span of nine years, it had managed to reproduce and spread far throughout various locations that are yet undiscovered and unoccupied by the Combine.

Somewhere far into the Wastelands, the unknown alien micro-organism takes over a large area by contaminating it entirely with a deadly radiation. As the Combine were setting up defense fields across the Wastelands, they have discovered the largely contaminated area. This led to them sending clean-up units to decontaminate the irradiated area.

However, before they could have began cleaning up, Xenian Wildlife accidentally wandered into the irradiated area. As the alien creatures stepped into the radiation, they were unwary that they began to carry the unknown alien micro-organism through the form of virus-like cells. When the Combine returned to clean up the irradiated area, the contaminated aliens had ran away and began carrying a plague with them back into the open world.

After the Combine finished cleaning up the entire irradiated area, a few samples of radioactive substance have been saved for further examination while the rest were dumped into a different unoccupied location. One Xenian creature, known as a Headcrab, carries the plague with it into an abandoned underground tunnel. As it travelled further, it reaches an entry point leading to the sewer system of City 13, a Combine occupied city. The alien creature makes it's way into the sewers, not knowing that it could be ready to unleash hell on Earth for the second time.