Walk into a room with ten people in it on the Citadel and you'll hear twenty two different words for them, fourteen of which aren't repeatable in polite company. Protoform, Corpomorph, Genestealer. Call them what you will, the odds are that one of the ten people in the room is one himself. They are the perfect soldiers, the perfect spies, the perfect killers. Able to reshape their bodies to anything they could need to further their goals. And the worst part? If they succeed, twelve billion of them are waiting to be unleashed on the galaxy.

The Council had found the planet Earth in the year 2067, according to their timescale. The ruins of a civilisation filled with shapeshifting monsters fighting over what remained. From what little the first contact teams were able to ascertain, an individual named Mercer had spread a virus among the entire species that infected them, turned them into zombies for want of a better word. Then another individual, Heller, had broken into the building where Mercer's sister was being used as the nexus for the hive mind and injected her with a formula that mutated the virus into the strain that transformed every infected member of the species into what would become largely known amongst themselves as the Evolved, or the Protoform.

When the first contact team landed, they were greeted by almost two hundred Protoform, curious about the great metal object fallen from the sky. The contact team had requested more personnel and resources and the orbiting fleet had been more than happy to comply. Then the second wave had requested yet more groundside personnel, including significant security forces. Once again, they had received their wish. Then at some unspoken signal all contact teams had withdrawn to the ships and requested to be returned to their respective home planets. Almost as soon as they disembarked at their home ports they simply ceased to exist. Then people started disappearing. By the time the authorities figured out what was going on the two thousand or so people that had set foot on the Protoform planet had translated into almost half a million disappearances across the galaxy. The planet was almost instantly quarantined, what was unmistakably a Protoform recon force cut off from their home. Their ultimate objective was to deliver the rest of their kind off their desolated homeworld. If two thousand Protoform caused such a swathe of disappearances the Citadel races shuddered to think what twelve billion would do.

It was estimated that the Citadel itself was home to anywhere between twenty and four hundred Protoform. Special Operations Officer Garrus Vakarian was charged with hunting them down. In his three year tenure as head of the Protoform Task Force, his unit had apprehended exactly one of the shapeshifting aliens, who then proceeded to blow apart the entire precinct with a sunburst of razor sharp tentacles before Garrus put her down himself with a sniper round through the eye. Sometimes his job felt like it would crush him. When he got that sense he would think of all the people that one Protoform he killed would have consumed had his team not acted when they did. The lives of twenty nine officers had to be worth something.

He brought the thermal binoculars to his eyes, watching the activity inside the apartment his men were staking out. They had a suspicion one of the apartment's three occupants may be a Protoform, and more than that he was hoping that his team could pressure the thing to make a move.

The Asari matron bobbed her head to an unheard beat as she prepared breakfast for her Drell bondmate, who was bending over the cot containing their year old baby daughter, no doubt making those strange sounds people made around babies. The infant reached up a pudgy hand to her father, who picked the girl up and jogged her up and down a couple of times. Tiny arms went round his neck and squeezed. Garrus couldn't help but smile at the sheer domesticity of the scene.

The baby child squirmed slightly and the father put her on his shoulders, swinging her round slightly. Garrus's Omnitool beeped and he glanced down at the display.

[POSITIVE VIRUS TRACE]

Garrus frowned. The portable detectors could only pick up vital traces directly before a Protoform shifted - He looked at the father and child with dread.

"Oh spirits no."

The child on the Drell's shoulders suddenly got bigger, thermal signature increasing in size as the father seemed to fold into her body, a body that took the exact same form as the man she just consumed. Garrus was dimly aware of one of his men screaming in disbelief.

"No way! The kid was the Morph!"

He jammed his finger on the Alert button.

"Confirmed Morph! It's the Drell, Go Go Go!"

The C-sec gunship skimmed over the rooftops, coming to a halt above the apartment as his highly trained kill team rappelled down onto the apartment's roof, busting in from above even as the other half of the team surged in from below, cutting off ground escape for what little good it did. He forwent his thermal binoculars in favour of the thermal scope of his heavily customised sniper rifle. The Protoform ran into the kitchen and tapped the Asari on the shoulder, exchanging some quick words. Then the both of them reached under the kitchen counter, pulling out long barrelled assault rifles. Their bodies shifted, transforming into the nigh impenetrable armour all the Morphs seemed capable of producing. Garrus swore under his breath.

"Two confirmed morphs! Repeat, two confirmed morphs!"

Garrus itched to be in the action with his men, but he better served them here, behind his rifle. Both Protoform opened up with their assault rifles, shattering the floor to ceiling windows of the apartment they had occupied as they took off into a sprint straight for the new exit. Garrus sighted down his rifle and squeezed the trigger, but operating on that uncanny sixth sense the Morphs possessed, the one he had targeted brought up a shield, the antitank round shattering the ablative device but leaving the Protoform unscathed.

The targets took swan dives out of the building, long wings erupting from their backs. The wings were a relatively new thing and seemed to only be used for short distance glides between buildings, but the fact the Morphs were improving wasn't a comforting one. Hard vacuum seemed very little impediment to them, as did extreme heat, cold, electric charge, pH or extreme mass effect fields. In fact, the only thing that seemed capable of hurting them, ironically enough, were bullets and even then an awful lot were required. Fortunately, the gunship's chin mounted chaingun went a long way towards the requisite volume of fire, targeting the relatively delicate wings. It didn't do much good - the Morphs quickly alighted on the surface of the next building over, shapeshifting legs flash-forming long hooked spikes that enabled them to quite literally run up the surface of the building. The chaingun on the gunship reluctantly quietened down, unwilling to decompress a building full of civilians just for the sake of putting bullets into a couple of Protomorphs, who quickly crested the rooftop.

"Stay on them! You three, with me!"

The three C-sec officers on the roof with him, two Turians and an Asari, followed him at a full sprint to the waiting patrol car.

"Quick, get in! T'Sulen, take the controls!"

He glanced around as he swung himself into the car. The glance saved his life. He dove out of the vehicle just in time to avoid being crushed by the enormous block of matter one of Officer Venetia Rantarius's arms had become, hearing the patrol car getting pulverised halfway through his roll. By the time he had drawn his pistol the third Morph was gone and T'Sulen was screaming and clutching at the stump of her wrist, her hand still clutching her pistol as it lay on the floor two metres away. He rushed over and applied medigel, ensuring she wouldn't bleed out or die of decompression, then turned to the other officer, Scoriolanus Metacian. To his surprise he found himself staring down the barrel of a pistol.

"Metacian, what are you doing?"

The wild eyed young recruit's hands were shaking badly, so much so that there was at best a fifty fifty chance he'd hit Garrus if he fired, and his kinetic barrier would protect him long enough to go for cover.

"It could be you! Huh? How do I know you're not a Morph?"

Garrus immediately drew his own pistol, aiming squarely at the rookie's right eyeball.

"I could ask you the same thing, Rookie."

He had seen this plenty of times before. One of his officers would accuse another of being a Morph, only for the accuser to break out the claws and hamstring his men while they were busy pointing guns at their fellow. Apparently the Morphs called it 'patsying'.

"It's not me! I'm pretty sure I'd know if I was a Morph!"

"I'm pretty sure you wouldn't tell me either."

Beside them T'Sulen groaned.

"Let's work out who's a Morph later. She needs our help."

Reluctantly the rookie lowered his pistol, Garrus following suit a moment later. The Asari officer whimpered again, seeming unable to focus on anything save her stump of a wrist.

"T'Sulen, can you hear me?"

She didn't reply for a second. Then her missing hand was abruptly replaced by a set of long claws, claws that buried themselves in Metacian's gut.

"Surprise, motherfucker."

Metacian's corpse was swung at Garrus faster than he could comprehend, tearing in half through the sheer stress it was subjected to as it slammed him into a wall, winding him. The creature that had taken T'Sulen's form advanced on him, claws scraping against one another ominously.

Garrus was certain this was the end. He had always known he would be killed by a Protoform, ever since he took the posting, but he hadn't expected it to be quite so anticlimactic. He had expected a running battle through the Presidium, car chases and gunfire and explosions for him to eventually sacrifice his life to save a busload of hot Asari dancers. Not this, dying in ignominy on a rooftop at the ass end of Zakera ward.

The Protoform turned, then sprinted for the edge of the rooftop, leaping off as a C-sec gunship appeared above the roof. Garrus let out a breath and slumped back. That was close.