The Flood are now going up against the Combine in the Half-Life universe.
I'll be honest, this isn't my best work, but I hope you enjoy this one-shot.
As always, I do not own Halo or Half-Life.
In another universe, on the Combine Overworld, workers of the Universal Union bustled to and fro, determined to complete their tasks. Hordes of Combine Overwatch forces patrolled the interior, exterior and airspace of the vast Citadels that covered the planet.
The Combine Advisors looked on at the empire they created, proud of their accomplishments. The activity never ceased, it could not be allowed to. The heart of the Universal Union never slept, and it was (as some would readily admit) a model of efficiency.
At that moment, the efficiency came to a grinding halt. A superportal opened in the skies over a cluster of Citadels, black energy surrounding a blood-red core. This was unexpected to the Advisors, as they were always informed of when and where a new portal would be created. Furthermore, their portals were never on this scale. The Advisors activated a link to several of their gunship squadrons, allowing a clear view of the unexpected rift.
And the horde of abominations pouring through it.
Combine gunships were the first units that emerged from the portal, but they were covered in green and brown tumors and had similar colored and textured creatures hanging from the top and sides.
Hundreds of gunships unleashed dark energy bolts and Combine fortifications and synths, as hordes of squid-like creatures leapt from portal and attached to Combine synths, corrupting them into more of these combat forms.
Millions of infected soldiers from species both known and unknown to the Combine were also flowing through portal, carrying unbelievably powerful weapons which were tearing apart Combine military forces with disturbing ease.
The brutal conflict that followed defied most forms of understanding. Miniature singularites opened in the middle of combat hordes, consuming them in a nanosecond. Beams of energy tore through the hardened Citadels like a knife through butter. The devastation to the Overworld was immense, and the flood of hostiles showed no signs of slowing. For every Combine synth that died, their enemy gained a new combatant for their side.
Strider and Hunter combat forms rampaged their way through the Citadels, while aerial synths duelled with aerial combat forms. The rate of infection by these creatures was unnaturally fast so the Combine Advisors ordered their subordinates to contact the rest of the empire to send reinforcements to crush this infestation.
They found that their communications had been hacked and severed by these abominations, and shortly thereafter, the first advisors were infected by these monstrosities. With the newfound knowledge of the Combine's inner workings, the parasite began overriding the portal controls, trapping the forces of the Combine Overworld and corrupting them at their leasure.
7 hours later, it was all over. The capital of the Universal Union had fallen, with the rest of their empire set to crumble shortly afterwards. Oh, the Combine would not go down without a fight; it wasn't in their nature.
But since the heart of their empire had been taken over by the unexpected appearance of the Flood, the rest of the Universal Union was blissfully unaware of the sheer hell that was about to rain down on them.
On the precipice of the ledge on one of the highest Citadels, a square portal of white light opened and a what appeared to be a human stepped through the entrance. Dressed in a Cold War era suit and tie, and holding a briefcase in his right hand, the man calmly walked to the edge of the balcony and observed the carnage unfolding with a neutral expression on his face.
To the uninitiated, he could be mistaken for a human, but to anyone who knew better, several aspects marked this man as unnatural. In addition to the aforementioned unnatural calmness to the events unfolding in front of him, there was also the fact that his face was weirdly symmetrical, and his mannerisms were disturbingly wooden.
As the man observed chaotic infestation, a cluster of Flood tentacles slithered up behind him. Taking notice, the G-man turned to the encroaching tentacles as they slowly approached him.
"It seems our... deal is complete." The G-man said to the tentacles. He had agreed to give the Gravemind access to the Combine Overworld in exchange for the entity to cripple the Combine.
"So it would seem." The Gravemind's tentacles prepared to coil around the G-man when suddenly, all of them froze in midair.
"My employers... agreed to this for our own ends... not yours. We... are beyond your reach." The G-man stated. The tentacles retracted away from him. Satisfied, the G-man turned, reopened a doorway and started to walk towards it.
"Nothing is beyond my reach." The Gravemind said, halting the G-man in his tracks.
Flood infected Vortigaunts teleported in behind the G-man and other Flood infected eldritch abominations began appearing in the sky over the Combine Overworld. He turned around to face the tentacles and allowed the Gravemind to address him.
"Your employers better pray that I never find them." The Gravemind threatened, "And when I do, I'll be coming for you."
The G-man felt a wave of fear course through his essence, and though there was no outward sign of his apprehension, the Flood already seemed to be amused by his pure terror. Straightening his tie, a wave of anger coursed through his voice.
"We'll see... about that." After that, the G-man turned back to the portal and left without another word.
Sorry that it's taken me a while to release another chapter, but the spark for this story has kind of left me, so I think I'm going to take a break from this collection for a while. I am by no means done with this, but new chapters will not be coming out as often as they were before.
To those people who requested franchises that I didn't get to, I do apologize and hopefully I'll get to yours as soon as I can.
In the meantime, I'm working on another multi-crossover called Desolation, so give that a look if you have the chance.