Mass Effect is the property of Bioware, Alien is owned by 20th Century Fox

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USCSS Calypso

Asari lived a long time. Barring injury or illness, Shiana could easily expect to live another six or seven centuries, though, at this moment, that seemed to have become an unlikely prospect. Even so, Shiana knew that no matter how long she lived, she would never forget the creature that now stood before her. It wasn't the fact that it was ugly. Krogan were ugly; so were Vorcha. For that matter, she had never found Batarians or Elcor particularly attractive. This creature was far worse than merely ugly. It was pure menace. Dark green, almost black, it was about the size of an asari, though its crouching posture made it difficult to be sure, with a a wiry, almost skeletal frame that seemed encased in some sort of natural armour. The arms were long enough to reach the ground and it had a long tail, covered in spikes. However, the most terrifying part of the creature was its head: a large, smooth, armoured dome that seemed to lack any kind of features. Seemed, because even as that thought went through her mind, the creature opened its mouth and showed that it had at least one very clear feature: teeth; and more teeth and behind those, yet more teeth.
For one terrifying moment Shiana stood as frozen, while her mind tried to comprehend the nightmare creature in front of her. It took a step forward, its tail lashing back and forth. The movement broke the spell and Shiana felt the surge of power flowing through her body, as, more on instinct than by design, she struck with her biotics.
The blueish glow of her warp rippled over the creature. It staggered back, snarling, and the inner set of teeth suddenly snapped forward, protruding far out of the drooling mouth. A moment later it had recovered. Although the warp might have stunned it for a moment, it was clear, that the thing would not be so easily defeated.
Shiana raised her submachinegun. She had never been a particularly powerful biotic and usually put more trust in weapons to do the fighting for her. The first burst hit the creature dead centre and she could see a green-yellowish fluid welling up from the wounds.

I can hurt it! If I can hurt it, I can kill-

The creature twisted sideways, a nearly impossibe feat in the narrow corridor, its claws grappling the wall and ceiling as it twisted to avoid the bullets and Shiana felt her despair return. Her Tempest submachinegun was designed for extremely rapid fire to wear down enemy shields and biotic barriers. It was difficult to control on long bursts and the individual bullets didn't do much damage.

I'm not going to make it. It's too fast. It-

The creature jumped forward with terrifying speed-
-only to to stop in mid-jump as a biotic singularity flared up in the middle of the corridor. Looking over her shoulder, Shiana saw Liara T'Soni covered in the blue glow of her biotics. Sanity finally returned and Shiana dropped to one knee to give her two remaining teammates a clear field of fire.

"Shoot it! Just Shoot!"

The withering fire from a submachinegun and two heavy pistols tore into the creature, even as it struggled frantically to fight free of the singularity. It shrieked as the bullets ripped through its head and torso. For a moment, it seemed as though it might actually survive long enough to break free, but, just as the singularity faded, the creature's bullet-riddled carcass fell to the deck, still twitching.

"By the goddess, what-"

A hissing noise interrupted T'Soni's shaking voice. Steam rose from the area around the corps. Where the deck had been splattered with the creature's blood, the plating, already buckled by the singularity, seemed to melt and before the trio's unbelieving eyes, the corpse started to sag through the deck.

Acid! That thing bleeds acid!

"Fascinating." Mordin Solus pushed passed her to get a better look at the corpse. He opened his mouth, then closed it again. For once the salarian seemed at a loss for words.

"Alright, that's enough. Back to crew compartment. Now!"

UMS Anzio

Anderson took his seat aboard the Viking dropship and placed his personal weapon in the holding clamps next to his seat. He patted it affectionately. The new Lacrima 99 assault rifle had come into service quite recently, replacing the ubiquitous M41 series that had served the Colonial Marines for the better part of a century and was still in use among many local militias. Unlike many other weapon systems from that era, which had been selected more for their potential to fill the coffers of various corporations than their battlefield performance, the M41 had actually been a popular weapon, and no-one had been in a hurry to replace it. Still, by now even most die-hards had been won over. Like its predecessor, the Lacrima combined an assault rifle with a grenade launcher. However, their position had been reversed, with the grenade launcher mounted above the rifle. With the grenades fed from a bullpup-style magazine, this allowed for a much longer barrel, improving the accuracy of the grenades. Moreover, the grenades themselves had been redesigned, allowing for both direct and indirect fire as well as a variety of payloads. The assault rifle had a shorter barrel, of course, but that was acceptable, and specialized ammunition made up for the reduction in firepower. That thought reminded him:

"Everyone! Final check. Make sure that you have frangible rounds only. I don't want some idiot firing armour-piercing rounds inside a spaceship. We all have to die sometime, but I'd rather it wasn't through stupidity."

There was some laughter, but it sounded strained.

Zim is right. They are too tense. One wrong move and this will end in a bloodbath.

Anderson tried again.

"Everybody ready to meet the little green men? Come on, we're about to make history. You can at least pretend to be enthusiastic. Think of the stories you'll be telling your grandchildren."

"Hey, maybe they're actually hot, green, space-babes!"

Thank god for O'Connor, you can always trust him to lighten the mood.

"What did I say about watching cheap SciFi, O'Connor?"

"That it will rot my brain, sir."

"I guess my warning came too late."

This time the laughter appeared a bit more relaxed.
By now all of he marines were seated. Even the machinegunners had manage to stow away their heavy smartguns, an especially awkward task in the cramped confines of the shuttle. That only left the civilian.
Anderson took a look at Vickers, clearly recognizable in her civilian style environmental suit. She had put her briefcase in a locker and was just now sitting down, struggling to secure her shotgun. Unfortunately, it had not been designed to fit in the standard clamps. It was a short weapon with a heavy barrel and a drum magazine behind the pistol grip. Compact and capable of rapid fire it could do a lot of damage in close quarters, making it ideal for this kind of mission. Finally, Vickers managed to get it properly secured.
Anderson took one final look around, then he activated his comm.

"Pilot, this is Anderson. Inform the captain, we're ready to deploy."

USCSS Calypso

The boarding team, what was left of it, had returned to the bridge.

"Was it... Do you think it was part of the crew?"

Shiana looked at T'Soni. The younger Asari seemed to have recovered somewhat and there was no denying that, terrified or not, she had done more than her share during the confrontation in the corridor. Perhaps she was one of those people who performed best when under pressure. Her question was also extremely valid. Unfortunately, it was not easy to answer.

"I don't know. It seemed more like an animal to me, but then again, so do a lot of Vorcha. Solus?"

The salarian, too, seemed to have recovered, and it was clear that he had been pondering the implications of the creature's appearance.

"Unlikely. Species clearly different from corpse. Claws too large for instruments on bridge. Tail: awkard for chairs. Also, highly evolved predator, highly specialised. Unlikely to become tooluser." He considered for a moment. "Might be pet, or trained to fight, like Varren. Could be very effective, if properly deployed. On the other hand, might be what killed crewmember. Not sure about method, but certainly seemed capable."

Shiana had to agree, still...

"Even if it's a trained animal, I don't think I want to meet anyone, who can train that thing. It makes Varren look positively benign." The Tuchanka born predators were commonly found among Krogan mercenary groups, both as an extension of the fighting force, and as a source of entertainment for the krogan, who made a sport of betting on Varren fights. Still, they were known to occasionally display a form of affection for their owners. This creature, on the other hand... She closed her eyes for a moment, trying to purge her memory of the image of the hideous thing they had just encountered. "I think we know what happened to Donal and V'Rini. That means that there are probably more of these things on board. We cannot risk the corridor again. If another one, or, goddess forbid, more than one, catch us in such a cramped space... I'll contact Lucinus. Have him take off and match position with one of the airlocks on this side of the ship. Even if he cannot properly dock, we can open the airlock from the inside and jump to the shuttle. "

She sighed.

"We have to accept that Moryala, Donal and V'Rini may be gone. That thing didn't look like it was the type to take prisoners. In any case, there is nothing we can do for them from here. Once we're back aboard Light of Dawn, we'll consider out options. Whatever that thing was, like you said, Solus: it didn't look like a tooluser, so a properly equipped commando team should be able to handle them, as long as we can keep them at a distance."

Het omnitool chimed as it registered a incoming message.

"Yes?"

"Captain?" It was Lucinus. "Message from officer Levarn, ma'am. It looks like you're about to have company. There are two large shuttlecraft underway from the new ship."

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It's ME aliens 1, xenomorph 3.
I tried to strike a balance between ME style fighting and the way xenomorphs work in both the Alien movies and the video games. Singularity, followed by lots of gunfire, seemed a good tactic, because it both neutralises the chestburster's speed and contains the splatter from the acid. Of course, Liara didn't know about the acid in advance, but I figured that a singularity was the logical thing to do for someone who is absolutely terrified, and desperate to keep the creature from coming any closer. Besides, they've already lost half of their boarding team. They deserved a break, so I let them find out about the acid the easy way.

I'm using real life prototypes for the human guns. Anderson's assault rifle is basically the XM29 OICW (I borrowed the Lacrima name from Alien Resurrection), while Vickers' shotgun is a Pancor Jackhammer Mark 3 (It looks futuristic and actually resembles ME shotguns a bit). Still looking for a good image for a replacement of the smartguns. The Tempest submachinegun is, of course, from ME2/ME3. I didn't identify the pistols used by Liara and Mordin, but those would be Predators.

Once again, thanks for your comments.

As several people suggested, using images would indeed be the best way to handle first contact (assuming the aliens have eyes). Of course, you still have to overcome electronic problems unless you actually meet in person. That's why Levarn is working on establishing a simple bit-map configuration that both sides could read. If the humans can figure that part out, it will make things easier. Still, he will have to broadcast the bitmaps basically in Morse-code. That may work for simple images, but for anything more detailed, you need something better: AM/FM, broadband, anything to get more information across. That means they have to match the two systems somehow.
Vickers is actually carrying a laptop in her suitcase with a number of presentations to show to the aliens once she is aboard the Calypso. Assuming, of course, that they don't get shot during the boarding action.

MEEKVERSEFAN2.0: I don't think krogan would be completely immune to the facehuggers, but they would represent a serious challenge. If you restrained the Krogan, the facehugger should be able to implant the embryo. Under normal conditions the size of a krogan's head might be a problem, since the facehugger couldn't cover it completely. Also, I think a Krogan would be strong enough to rip the facehugger off, and tough enough to survive that procedure. After all, Ripley managed it in Alien Resurrection and I don't think she was as strong as a krogan, even with the alien DNA in her system. Cutting off a krogan's airflow wouldn't kill them any time soon (In ME2 Wrex seemed to consider being spaced survivable.) So, I guess the outcome would mainly depend on whether the facehugger can put the krogan in a coma in time to avoid being torn apart.
Interesting idea, though. I shudder to think what a krogan based xenomorph would be like.