For those of you not familiar with Homeworld:Cataclyms -
Deacon-class Destroyer,
The Beast. I think the story is pretty well self-contained in itself, though, ie you shouldn't have to read those to understand what's going on. A basic understanding of Sci-fi should suffice.

It was a dark and stormy night.
That is, according to ship's time it was night. The phosphorus nebula the destroyer
Shuunusu-Do inhabited hardly had any concept of night or day.
They hunted the Beast.
Unlike the ship's time, this was an uncertainty. The sensor capabilities of the Beast were unknown. they had infected many ships across this quadrant of space. Whether or not they- it had absorbed the advanced sensors mounted on the
Deacon class of destroyer vessels, no one could be certain. Whether it had even now outmaneuvered the lone destroyer and was closing in for the kill, no one knew. All they could do was wait – and hope.
Their specific target was that of a ship that had split off in the middle of a battle. Fleet Command suspected its objective was to infest unsuspecting or lightly-guarded ships, and then return in greater force once the defense fleets in the quadrant had decided that the Beast was eradicated.
Ships were falling back across the line; in this quadrant, oddly enough, the Hiigaran vessels had gained the upper hand. Some had postulated that the primarily agricultural economic base in the quadrant made it of little value to the beast; still others had postulated that the highly-structured biomass of the crops would easily be adapted by the Beast to serve its own means.
And so the
Shuunusu-Do glided through the nebula, ever vigilant. The lightning that shot across to touch the hull made the crew thankful for the advanced EM shielding.
At their stations aboard the vessel, sensor officers quietly muttered to each other, attempting to confirm a contact. The captain of the vessel sat tensely, somewhat stressed from the time spent hunting the enemy destroyer and the lack of sleep.
What none of them realized was that it had all been a ploy. The Beast ship had jumped out of the nebula long ago. The destroyer was alone; but the crew was not.
During the battle, an infected cruise missile had been detonated by nearby sentinels shortly before it had impacted the destroyer's hull. A very small portion of Beast biomass had managed to survive, however, and impacted the hull, grasping on to the minute protrusions and imperfections left by the repair crews. It slipped in through a cracked heat sink's casing, making its way to an obscure control pathway, partly by chance, partly by its core programming directive. Once there, it slowly drew sustenance from the nearby walls, growing in size from the very elements meant to fight it, splitting off a portion of itself to regulate any alert messages. It then began to take control of simple electronics in the area; door openers and light brightness regulators, water filtration units, even the internal sensors. Of course, they all sent messages of failure; and every one of them was intercepted and discarded.
When the spore had determined it had a viable target, it sent a message to the Self controlling the fleet. Based on its collective knowledge of Hiigaran psychology, Self ordered the nearest destroyer to depart, but in such a way to make the Hiigarans believe it was for some intentional purpose. What that was did not matter; merely that the destroyer followed. It carefully ensured that the vessel could be tracked to its destination, and ordered it to jump.
The first victim of the Beast was an engineer who had the misfortune of going on an errand that took him through the corridor. His first clue that something was wrong was that the door refused to open. The controls were fine, and, being an engineer, he decided to fix the problem that must have occurred in the door controls on the other side. He commed in that he was correcting a systems malfunction, and that someone else should inform the flight officer that one of his pilots had been tinkering with their nav systems.
Now happily engaged in something other than passing on trivial messages, the engineer Tresh'nu popped the emergency access panel and started cranking the wheel that should have opened the door; oddly enough, it wouldn't budge. Figuring that the subroutines controlling that particular door had frozen up somehow, being close to the outer hull and the EMP-heavy nebula beyond, Tresh'nu brought out his portable diagnostic computer and sent an override to the door control.
On the other side, the organism suddenly sensed the override attempt. It reacted strongly at first, beginning to center itself upon the point of intrusion. When it realized the true source, thanks to the commandeering of surveillance information, it began to play the part, sending data to the diagnostic device and the door control's indicators that meant a successful restart. By now, those controls were simply puppets of the beast; it was a simple matter to subvert them to its uses.
And so Tresh'nu entered the corridor. His first indication that something was wrong was that the lights were not functioning properly; some were at full power, others were completely dark, a ploy by the Beast organism to maintain the power levels which would be used for the partially-dimmed night lights.
His second indication that something was wrong was that the door slammed shut, all the nearby lights went off, and something fell on his face, over his eye; it was room temperature, soft, but it made his skin itch.
There was no third indication. By the time the of the body's first spasms, Kresh'nu was already dead, the brain of the body massively hemorrhaging as the beast organism shot into his nasal passages and stabbed the brain, searching for the control it needed. By the time the body started to scream, the Beast was taking control of gross motor control. By the time the body hit the floor, the Beast had begun to recreate the fine motor control necessary for what it meant to do.
The Beast soon had access to the last memories of its host, decrypting the brain's own storage mechanism and substituting it for its own. Regrettably, it was unable to achieve the efficiency it desired, due to the limited space available, if it wished to maintain the outer appearance of its host.
It pushed itself off the floor of the deck with its forward control appendages, quickly recalibrating its balance routines to maintain bipedal movement, which the memories it had accessed indicated a preference for. There were some other small movements that were performed, but unfortunately most of that information was lost in the initial transference.
It began walking through the corridors, towards C deck, passing only a few crewmembers. They noted the odd movements of the body, the lack of movement in the arms, the oddly unmoving eyes, but most did not care. One inquired what was wrong, and the Beast sent back a single, monotone reply - “Nothing.”
Finally, it arrived at its destination; the barracks room, where much of the crew slept. The night shift was truly advantageous to the Beast, it realized, and it reflected that it was unfortunate that it had not realized this odd state of the vessel's awareness. The next stage of its plan would be less subtle, therefore, by necessity, but with many more Selves it would be possible to accomplish.
It shut the door and turned off the lights, extruding a small part of itself to take control of the door mechanism and any monitoring objects in the area. It moved over to the area where this organism had operated during its recovery cycle, awkwardly attempting to operate the small console there with the extremely primitive interface devices of the host. Soon, it was pleased to note that these inefficient collisions were no longer necessary; it stood up and walked over to the body for its next Self, hoping that it would be able to control this one with much less destruction than its current one.
57.3% of the way to completion of its goal, a slight difference in the location of brain pathways caused one of the crew members to shriek; as a failsafe, the Beast developed a new method of ensuring that host bodies did not emit noise, by placing one of the appendages over the oral passages of the host prior to its defection. The other selves acknowledged this method, and no other accidents followed.
On the bridge, the security officer noted that one of the engineering offices had emerged from their bunk room, and that the daytime support staff was following. As the crew was distracted by this, the Beast elements infesting the original control pathways began to rewrite code and spread itself; several minor system glitches followed. But the bridge crew, occupied with or distracted by the odd behavior of the crewmen, failed to notice or attach any real importance to these issues.
One of the selves discovered, in the darkest regions of its host's mind, a fear of eternal nonexistence – a system that the leaders of the ship could apply to destroy portions of its crew, selectively terminating any personnel that had defected. This containment method had a weakness; it was controlled by one central location, heavy in the equipment that the Beast savored. Further, one of the crewmembers that had become a Self could apparently enter the bridge without much disturbance or alarm.
This One was selected to carry out the crucial part of the plan. The controls were separated from the main computers that neutralizing without alarm would be impossible for the Beast; as were the self-destruct mechanisms that might be used as a last resort.
Therefore, defection by carrier was selected as the method of control. The Selected moved to the control location, while the other Selves moved to the scattered areas throughout the hull that crew were most concentrated in; several relocated to the mess hall, others slowly relocated to stations. All were careful to maintain minimal contact with personnel, to avoid being discovered.
Once the Chosen Self had reached the control section, it had converted a large portion of its host biomass to a new configuration that favored infection. It interfaced with the authorization device and entered the room.
The Commander turned. “What in Hiigara's name are – What in the Gods' names!?”
For an instant, the Captain stared at the head of the Day damage-maintenance team. His eyes were gone, replaced by red sacs; his ears had been replaced by gaping holes, within which portions of the brain – interspersed with circuitry – could be seen. The mouth was lipless, and the nose was also gone. The uniform he – but by now the Captain thought of the being before him as it – wore was interspersed with the flesh, now grey in areas. Behind the being, the two guards outside the door were jerking.
He pulled out his sidearm, firing rounds into the being while ordering the self-destruct primed. The crewmen at their stations got as far as pressing the red button that every single one of them had hoped never to press, except in the twisted training scenarios. Then, the consoles exploded, killing or severely wounding many of them. So preoccupied were they with their consoles, which had suddenly come alive, none of them ever looked back, with their own free will, at the being that had entered the bridge. Instead, they screamed in horror and pain as they lost control of their own bodies. Some were lucky; the infection form reserved for them landed on their head, or spinal columns, and they lost consciousness instantly.
Others were forced to watch and feel their arms and legs stop responding, and slowly start moving towards their head, just before they blacked out forever.
The last person to see the Beast entity standing just inside the bridge door by the captain. The Beast had preserved one unnecessary facet of its host – the oral muscles. It used these to recreate one action it had learned from its host. It smiled. The effects of this obvious indication of superiority would be analyzed once it had taken control of the inferior being standing before it. Then, it executed this; it ejected the thousands of infection organisms it had secreted in pockets across the body throughout the bridge.
In engineering, mess halls, damage control stations, barracks, the same event occurred. The beast's numbers grew quickly.
On the bridge, the Security offer who had been the first to notice the odd behavior of the crew had been leaning away from his station to see the conversation between the Captain and the Day damage-maintenance head. When he saw what had happened, he had instinctively leapt to defend the Captain, although not even armed with a sidearm. He narrowly missed the first wave of infection, realizing what was happening as he arrived at the Captain's command chair. As the Beast entity exploded, he reached out towards the controls that could not save his life, and pressed them, with every hope that they would end it.
But nothing happened. The security officer was one of the quick ones; the wave of spores that impacted him, and his proximity to the Beast being, guaranteed that.
On the five-second failsafe timer, the self-destruct mechanism – with its simple, yet heavily shielded electronics – computed that only 72% of the bridge crew had acknowledged the count. It checked with its internal directives, searching for the one answer it was programmed to find. It determined the answer, and detonated, vaporizing all of those aboard, Beast and Hiigaran alike.
Recon units were sent to determine what had happened to the errant destroyer that never reported in. Months later, a destroyed Beast vessel was identified as its target. But the wreckage of the
Shuunusu-Do was never found.