Circumference engine room.
Kirrahe entered the engine room, scanning the room with his pistol as he checked for any sign of the entity. He'd turned off the radio after hearing the death cries of the Captain. He didn't have the time to mourn for him.
There were several human survivors at work fortifying a part of the room pistols, shotguns, and an assault rifle. They looked scared but determined. ONI, like the STG, did not recruit the meek. Kirrahe introduced himself by speaking from around a corner. "It's me, Kirrahe. I'm entering the room with a cloaking field. Don't shoot."
There was some muttering. "Do it slowly!" A voice yelled from inside the room.
Kirrahe stepped into view of the defenders, his cloaking field turning off and revealing himself. "The Captain is dead, and the bridge has been lost. We need to detonate the reactor immediately." Kirrahe walked towards the engineering console, connecting himself with his omni-tool and quickly scanning it for any method by which he could destroy the ship. "There is no time to waste. Whatever we are dealing with is a malicious entity from Slipspace." He said, turning to the engineers as he worked.
One of the humans walked forward, gripping a pistol tight. "We tried to prepare for self-destruction when the Captain declared Cole Protocol. But something in the system delayed our effort, then the lights went out and the systems started failing. Whatever is aboard the ship, it's got access to every system Leng did not disconnect from the bridge. When we tried to order a self-destruction, somehow, the thing started sending countering our actions before we even did anything."
"What do you mean?" Kirrahe asked quizzically.
"We tried to force self-destruction multiple times, but this… this thing somehow got all the disarm codes. I have never seen anything write code so quickly. It's writing programs on the fly to counter existing counter-intrusion measures."
Kirrahe instantly realized what had happened. The entity had locked down Leng by assaulting its database and then stolen all its data. That meant it had control over the ship's computer systems. It also had knowledge of how UNSC software was written, and was now making its own.
"Can we do it manually?" Kirrahe asked.
The engineer looked thoughtful for a moment, then turned to look at the engine. "We've been trying, but it won't be pretty. We're trying to smash the safety systems and flood the core with deuterium. The thing can set the reactor to emergency power all it likes, but it won't work without the regulating systems operational. But it still needs time to build up enough power to actually detonate."
"It's better than nothing," Kirrahe stated. He turned back towards the console, noticing a file transfer that had just arrived. It was the navigation data of the Circumference. He raised an eyebrow, then glanced around, seeing the engineers at work quite literally smashing the safeties off of the reactor housing. Kirrahe discretely copied the navigation data to his omni-tool and set it for an automatic full wipe in the event of dismemberment or death. He then removed the navigation data and crushed the data wafer it was on. A human turned to look at the noise and nodded, then returned to work.
"Barricade the doors and weld them shut. Clog the door mechanism with whatever you can find" Kirrahe said. "I'm going to lead the intruder on a chase." Kirrahe put his omni-tool on the stump of his right arm, and then prepared his pistol. It was a trusty Carnifex pistol he'd chosen after ONI had expressed disapproval of letting the Salarian take a venom pistol with explosive rounds aboard.
He left engineering and entered the darkened hallways of the ship, cautiously looking around him for any movement. He engaged his cloaking system and began the hunt. Kirrahe's eyes were fixed on the motion tracker of his HUD as he headed in the direction of the bridge.
It did not take long before he saw glowing blue eyes and flickering red sparks in the distance. It was the infiltrator with its back turned towards Kirrahe. It was plugged into a computer station. Kirrahe got a good look at what he was facing. The creature had shed most of its flesh, leaving nothing but bonelike glowing cybernetics that were imbued with a baleful light. As Kirrahe examined the tissue, he noticed that it had not been shed voluntarily, but been torn apart by high-velocity rounds. He assumed that the only part of the creature that it required was its metallic components, with any flesh being purely for infiltration. It had to be terrifying fighting an enemy that ignored its flesh being torn off, without slowing down.
The arms and legs of the intruder had become clawed appendages that could be used independently of each other, allowing it to run on all six of its limbs. Two smaller tentacles were attached to its spine, both outfitted with what looked like remote manipulators in the form of several clawed hands, each covered with fine cilia. Its shoulder blades looked like they had been replaced with crudely mounted plasma rifles that swivelled back and forth. The way the additional manipulators looked as if they could fold up quite nicely, and plasma rifles that looked like they'd been sunken into the creature's shoulder blades made him think that he was facing an infiltrator. While its skull had a targeting laser sticking out of the side on a small mobile tendril. Its jaw was a hideous bloody affair, consisting of massive jagged teeth and a powerful bite, which Kirrahe still remembered quite vividly.
The most substantial part of the being was its chest, which was heavily armored and as large as it had been before transforming, although there were several jagged cuts indicative of an armor-piercing round scraping by, as well as several dents, and even a hole. Thinking back to when Kirrahe had shot the creature in the head, he assumed that the head was merely a sensory system, with all vital functions being located inside of a hardened chest cavity. The chest looked somewhat like a suit of Sangheili armor. Kirrahe remembered the "trader" had been wearing a chest piece and robes. Either the chestpiece had been part of the entity hidden inside the trader, or it provided a good cover for a bulky chest.
Kirrahe decided to start by blinding the creature. If the chestpiece was the strongest part, it would be best to take away the advantages of the entity one by one. He switched his pistol to an incendiary-armor piercing double-tap.
He charged a cryogenic blast from his omni-tool and fired it at the head of the beast, then immediately switching his posture to lean his pistol on the stump. The creature turned its head in time to be splattered by the condensate. Its remaining flesh froze and cracked, while the metallic skull became more brittle. Kirrahe fired, putting two shots into the weakened neck. The first thermite round impacted the frozen surface and caused a small explosion of heat and steam, cracking the surface of the armor. The second armor-piercing round shot through the neck, causing large cracks that broke off the head from the body.
Then it turned the twin spine-mounted tentacles to face Kirrahe. They moved back and forth, then tensed up, aiming at him.
"Non-visual sensory system. Unfortunate." Kirrahe muttered, and flash fabricated a cryo grenade and threw it at the creature. It attempted to bat the projectile back at Kirrahe, but it detonated on contact, covering the abomination with the cryogenic solution. Kirrahe opened fire, rounds hitting but failing to penetrate the chest. Kirahe switched aim at the last moment, shooting one of the plasma rifles and knocking it off the mount. Kirrahe was about to smile when one of the two tendrils snatched the plasma rifle and took aim with it.
Kirrahe turned on his cloak and ran as fast as he could around the corner, feeling plasma bolts flying past, and hearing the clanging movement of the creature chasing him. He headed towards the hangar. Perhaps he could fight it in an open space? Or at least delay it long enough for the engineers to destroy the ship. He passed by dozens of corpses of barely armed crewmen who'd been killed by the infiltrator, trying not to be distracted by the grotesque ways they had died. The only thing that mattered now was to kill whatever he faced before it escaped.
His radio activated. "This is Sergeant Zaeed Massani. The boarding party is returning, ASAP. All survivors get to the damn hangar immediately!"
Circumference Hangar Bay.
Zaeed stepped off the Pelican, his rifle sweeping through the dark hangar. He had seen the power aboard the ship go off and heard the Cole Protocol be declared over the radio. Whatever had boarded the ship, he was ready to go in and kill the bastards. His marines advanced slowly behind him, more terrified of Zaeed than of whatever was inside the ship, especially after he'd struck a Marine who'd suggested that the Sergeant remain aboard the Pelican for treatment. Zaeed would have none of it. Nobody tried to enter his mind without getting a grenade shoved down its tailpipe.
The hangar was devoid of lighting, requiring the Marines to turn on their helmet lights set to maximum, and the lights on their rifles sweeping the area. There were no signs of life anywhere to be found. No crewmen or marines heading for the hangar to await the return of the boarding party. It was too damn quiet for Zaeed's liking.
"Set up lines of fire. Keep all entryways locked down. We have no idea what we're facing." Zaeed yelled, even as a medic was rushing beside him, trying to patch Zaeed's ruined right eye socket and frontal lobe up. He was convinced he would drop dead as soon as his adrenaline ran out, so he kept it up.
His radio chimed up. "This is Kirrahe. I am being pursued by the infiltrator. Prepare to ambush it. Use cryogenic and incendiary rounds and aim for the limbs. Its torso is too tough!"
"Kirrahe. Which entrance are you using?" Zaeed demanded.
"The main entryway. Wait. It just stopped its pursuit. It just leapt up into one of the air ducts." Zaeed cursed. It had to be listening in on the radio. "It might be trying to flank you. Get ready."
Zaeed looked at the marines and barked out his orders. "Double load: Triple cryo, triple incendiary." Zaeed cursed the lack of heavy weapons, but they were a prowler crew. They didn't carry anything more substantial than assault rifles. Omni-tools were lit, and weapon configurations were quickly modified on the fly. Each gun would fire three cryo rounds, followed by three incendiary rounds. All he and his marines had were assault rifles, shotguns, pistols, and grenades. No heavy weapons that could damage the hull.
Then Zaeed got an idea. His best one in a long time.
He walked up to the nose of the Pelican he had arrived on and began ripping the chaingun of its nose with judicious use of his omni-tool, coarse language, and adrenaline-fueled rage. With a final triumphant scream, he broke it off. Zaeed ignored the medics telling him to calm down, and began to jury rig a crude connection from the gun to his omni-tool. He worked frantically, the burning pain in his skull keeping him focused. He hefted the weapon triumphantly. His marines glanced at each other, not sure who was the most significant danger on the ship.
Kirrahe appeared, and to Zaeed's surprise, was missing his right arm. Kirrahe leaped forward and slid across the deck, flipping around and taking aim behind him as he did so. Zaeed had not seen the Salarian so panic-stricken before. He took aim down the doorway himself but did not see anything approaching.
"Kirrahe. What the bloody damn bastard hell is aboard this ship, and how did you lose your arm?" Zaeed yelled as the pain in his head wound spiked.
Kirrahe got up and looked at Zaeed's head wound, eyes going wide, but quickly got over it. "There is an acausal slipspace entity puppeting a cybernetic infiltrator unit aboard this ship, which is attempting to take control of the ship."
"A what!?" Zaeed asked incredulously.
"Spooky possessed robot killing people."
"Oh." Zaeed said. "Well. I've never killed a demon before. Guess there's a first time for everything."
"It's not a demon Zaeed, it's some kind of remotely operated cyborg which is controlled by something in Slipspace." Kirrahe said with irritation in his voice.
Zaeed glared at the Salarian. "It's something in another dimension that is possessing a body and wreaking havoc. That's what I call a demon."
Kirrahe sighed. "Whatever. Engineering is still alive and preparing to blow up the ship. We'll be going up in flames soon, but we have to stop this thing before it finds a way to stop them. We should hunt this thing down and kill it."
Zaeed nodded grimly. "Cole Protocol. But how do you propose we kill this thing?"
"I discovered its chest is too tough for most of our weapons to penetrate, but its limbs are weaker. We need to use cryogenic rounds to weaken them, then incendiary and armor-piercing to break its armor. Weakening it is a good way to start. It's how I removed its laser-guided electricity projector."
"The what?" Zaeed asked as he finished the final touches on his modified weapon.
"Lightning gun."
Zaeed nodded. "This thing is clever if it knew to cut the power and focus on the bridge. I'll bet my remaining eyeball that he'll attack the Pelicans if we leave them behind. I'll keep one squad on guard. We don't want him to get out of range of the blast for some other poor bastard to pick it up. You and I head for the engine room alongside my squad, and we'll kill the thing there."
Kirrahe nodded. "That works."
Zaeed spun up his chaingun, grinning wickedly at Kirrahe, licking the blood from his upper lip. "Time for a depleted uranium exorcism."