All There in the Manual: While many Mass Effect fics have in-universe codex entries to flesh out details that couldn't fit into the story proper (such as Protoculture Effect and Mass Effect Human Revolution), this story does it in a unique way- by having XCOM "Research Reports," showing both what XCOM's scientists can determine about the enemy by an after-action autopsy, but also what potential upgrades can be created out of them. For example, the Reaper Blackstar weapon is noted for potentially being able to be stuck onto a Blaster Bomb guidence package, vastly increasing the destructive potential.
Anyone Can Die: Pretty firmly established once Kaidan is rendered brain dead by the Thorian's psychic attacks.
Considering that it's a crossover with XCOM, it's pretty obvious that this would have happened eventually.
Nihlus was killed on Virmire recently as well.
Chellick the C-Sec detective died during the Battle of the Citadel as well. Jacob Taylor may have bought it also, but it isn't clear just yet.
Then again, as one reader noted,Nilhus' death was merely delayed from canon and Kaidan's ahead of schedule.
Word of God states that if anyone dies, they stay dead. No Lazarus project to bring back Shepard if he kicks the bucket.
The released outline reveals that the following characters would also have died: Jacob's death confirmed; Liara Indoctrinated and Huskified due to research on Sovereign's remains; Shepard paralysed by Balak, becomes first MEC Trooper but eventually dies self-destructing the Alpha Relay to destroy Harbinger.
Appeal To Audacity: The reason Shepard suspects that Legion is telling the truth about the geth heretics.
Asskicking Equals Authority: In Wrex's eyes. Both times Shepard does something Wrex disagrees with, he and Shepard have to throw down. Shepard wins both times.
Attack Drone: The basic unit used in XCOM's space battles are heavily armed and armored fighters deployed in large numbers to support the manned ones. There are also ground drones from canonical SHIV size to tank-sized.
The Battlestar: XCOM's supercarriers are an aversion, not being meant for direct combat despite their sheer size. They use psi-portals to deploy smaller craft in the field and their "main guns" are actually four one-kilometer long Fusion Lance/MAC hybrid mobile artillery ships.
Bad Ass: Shepard, Wrex, really the whole crew. Lets just expand that to all of XCOM to be safe, since even the OC XCOM grunts that didn't make it past Eden Prime were badass. They just ran into a Reaper.
Shepard: “You want to kill us? Fine. Give it your best shot. But know this. We will fight you, we will learn from you, and in the end, we will kill you.”
Berserk Button: Never say that Jack Harper was right in Shepard's presence. Just don't.
Councillor Valern suggesting that XCOM arranged Nihlus' death on Virmire has Shepard outright threatening to hang the Salarian Councillor by his own intestines. This is in front of the Citadel Council.
The Berserker: The "ogres". During the assault on Virmire, Shepard accidentally releases all of them. Thousands of enraged, superpowered tank-bred krogan-husks emerge and immediately start mindlessly slaughtering each other in a titanic brawl.
Big Friendly Dog: Rex the cyberdog. After being rebuilt as a half-ton heavily armed war-bot.
The tank-bred Krogan here aren't standard issue Krogan. They've had incredibly tough (and plasma-resistant) armor fused to their bodies, been implanted with some unknown tech that allows said armor to heal, and they're fifty percent bigger than regular Krogan. They also don't feel fear or pain. And Saren has an army of them. The team nicknames them "Ogres."
One of the specimens in the Virmire labs is a human child being Huskified. Shep delivers a Mercy Kill to the poor kid.
Creative Sterility: One of XCOM's issues. Give them something to reverse-engineer and they'll outdo the original, but start them off with a problem and ask them to come up with a solution... Nope.
Which isn't to say that they can't do it, it's more along the lines of that it takes much, much longer. Part of the reason why so much of the tech is very similar to that used in Enemy Unknown is because the humans have spent the intervening time perfecting what technology they do have, rather than trying to come up with something new.
The first contact incident between the Turians and Humans happens quite differently; because XCOM thrives on fighting aliens, and carries a massive fleet of drones armed with energy weapons and portal technology, the battle ends fairly quickly with few human casualties, and the Turians responsible for the incident captured. The battle goes so poorly, in fact, that the Asari have to rush in to stop the fighting before the Turians decide to get serious.
The codex entry for the coalition doesn't even give the conflict a name compared to canon's "First Contact War". After the Ethereal war, this was just another day for XCOM.
XCOM gets their own on Eden Prime. Nearly all of Shepard's initial crew bites the dust fighting only basic geth and the Beacon is destroyed.
Benezia vs Rachni Queen. The latter shatters the former's Barrier with ease and bites her in half.
Darker and Edgier: With the much more militant and paranoid XCOM taking the place of the System's Alliance, their interactions with the rest of Mass Effect's races are far more stark. The best example of this is when XCOM responds to Batarians enslaving humans by dropping chryssalids on a Hegemony world. In the political aftermath, the Coalition continues to play hardball with the Batarians by stating that they will only pay reparations if the Hegemony finally makes good on banning slavery, forcing the Batarians to leave the Citadel. In the end, this results in the near-total obliteration of the Batarian Hegemony, with most of the survivors fleeing to Citadel space as a civil war among multiple different factions ravages what's left of Batarian Territory.
Same applies to this fic's version of Cerberus. The original was a terrorist organization that wanted to improve humanity's lot in the galaxy. XCOM!Cerberus takes that Up to Eleven by instigating the attack on Mindior so that the galaxy will see that humanity is not to be trifled with. The Illusive Man is upgraded from a shady mastermind to a demagogue when he makes an impassioned speech defending his actions before his trial. Though, that's if you don't count Cerberus's actions in Mass Effect 3 and instead focus on their usual MO.
As this reader points out, The Coalition and the Systems Alliance have very different goals. The SA wanted to secure an alliance with the Citadel because Earth was trashed and they wanted help in their recovery. The Coalition, however, has no need for the Citadel or even for use of the relay network, as it is effectively "off the grid". Add to that the cultural scars of the Ethereal war and you have a completely different human political agenda altogether.
The Heretic Geth attack on Eden Prime leads to XCOM declaring war on the Geth homeworld of Rannoch, calling for their extermination. This version of humanity wastes no time on exacting revenge.
XCOM, according to Word of God, is meant to operate under these rules. They are always, as a rule, concerned that the latest alien menace will be the one to exterminate humanity. They are supposed to take massive casualties as a rule, and understand that they are selling their lives to save the human race. Even if they go down taking down an enemy, they know that in doing so they give XCOM time to build up their forces and utterly wipe out the enemy. It's very, very dark.
Pretty much everything from Mass Effect is given this treatment. The Thorian? Now the source of all psionics, and practically an eldritch abomination capable of shredding your brain- literally. The rachni? In this story, there were hardly any survivors at Peak Fifteen, whereas in canon there were dozens of survivors who were able to keep the bugs in check. And it turns out it wasn't some "Sour Yellow Note" that made them attack- it was that they were running away. The Krogan Cloning and Indoctrination facilities? Those are now upgraded to cloning horrible, muton-esque Ogres, capable of shrugging off even XCOM's vaunted weapons. Geth? They now have Vahlenite armor and biotics. Oh, and they can create black holes out of dreadnoughts.
Well demonstrated by the reaction of all present humans when Legion unveils that the Quarians reprogrammed the Geth to try to quell them. Since the Ethereal War, anything reminiscent of mind control is...not well accepted by humanity. The humans themselves are quite confused when Legion points out that the geth have no issue with it.
The Turian general responsible for nearly bringing the Citadel and Coaltion to war is affectionately named 'BlastHardcheese' by readers.
The Thorian, which is enhanced with psionic powers in this version (and supercharged beyond anything humans have seen before), was given the nickname "c’thauliflower" by some readers(a combination of C'thulu and Cauliflower). Agayek liked this name so much that he actually included it in the story.
Fandom Nod: '“I was hoping you could give me a crash course on how to optimize this thing,” Garrus said. “It’s doing pretty well so far, but I think it could use some calibrations."'
Fantastic Racism: In an interesting inversion of canon, it's Tevos (the asari Councilor) towards humans (she even calls Udina a pyjak), while Sparatus (noted for his canon portrayal as being the most antagonistic councilor) actually goes out of his way to defend human actions (blaming his own race for the First Contact disaster).
This makes sense in context: Despite generally being conflict averse, XCOM's very extreme actions and disdain for 'civil' diplomacy wears on her nerves, to the point that she can only see them as bloodthirsty warmongers. As for Sparatus, he like most Turians, have come to respect humans as warriors.
Flat "What.": Tevos gives one when told that a Spectre was present at Eden Prime.
From Bad to Worse: Repeatedly at the Battle of the Citadel. First Sovereign shrugs off the new weapons meant to fight the Coalition, then the impending reinforcements get locked out of the incoming Relays...
When the heretic Geth attack the Citadel, Sparatus authorises fire support missions, stating that the possible loss of thousands is preferable to millions or more if they get to rampage.
Also at that battle, Rufus chooses to go against his orders (Very Serious Business for a Turian) and deploy the weapons meant to fight the Coalition against the heretic Geth, figuring the risk of the Coalition reverse-engineering said weapons is worth taking.
Hold Your Chryssalids: The names of the Ethereal's servants are used in slang. Mostly as insults. Eg. Son of a muton.
Hypocritical Humor: Shepard calls the biotic Geth "cheating bastards". As one poster on spacebattles noted:
"Said by a reality-violating human space wizard wearing an armor from alloy that can only be made with space alchemy, wielding a weapon powered by green magic crystals."
Immune To Plasma: The "Ogre" krogan mutants are outfitted with armor specifically to defend against high-temperature direct-energy weaponry like XCOM's. They're not so effective against kinetic weaponry like mass accelerators or alloy cannons.
In Spite of a Nail: Saren is still Sovereign's tool in this version, he's just completely neutral about humans rather than condescending.
Insult Backfire: At one point, after Nihlus dies, Tali tells Legion that he "Lacks basic fucking empathy," due to some rather poorly timed comments by him. Legion then admits that, yes, he doesn't have any empathy, because he doesn't have emotions. He then states that he thought that this had been previously established.
Lensman Arms Race: The war between XCOM and the Geth ends up as a fight between who can out reverse-engineer the other. Starting with Eden Prime, the Geth adapt their armor to be more plasma-resistant. Later Garrus discovers that they've been stealing Titan armor from dead XCOM soldiers. On Feros, Shepard has the misfortune of discovering the first biotic Geth. Of course, XCOM gets its shot at adapting Armature weapons and synthetic muscles. Later on, Saren begins deploying mutated and cybernetic krogan called "Ogres" which can tank XCOM's infamous laser and plasma weapons with ease thanks to self-repairing thermal-shielding.
Lightning Bruiser: Wrex. Extremely strong and tough - even for a Krogan. Far faster than anything that big should be. Likes to jump straight into the middle of enemy formations while using his heavyplasma to hose down enemies and biotics to send anything that tries to fight back flying.
Ogres are strong enough to send Wrex flying through his Titan armour or One-Hit Kill with a punch right through kinetic barriers, shrug off plasma, yet are fast enough to surprise Wrex.
Madness Mantra: An indoctrinated salarian in chapter 22 keeps uttering "let me out".
Messianic Archetype: From the backstory, Colonel Abigail O'Connell AKA: The Volunteer. The first and most powerful human psychic to ever live (And used as the benchmark for psionic powers) and the greatest hero in human history. She's hit quasi-deity status by the 22nd century.
Mook Horror Show: The narrative switches to the perspective of the Turians at several points during their fight with XCOM forces in chapter 1. The descriptions of them being on the receiving end of psychic powers and plasma weaponry is...unpleasant to say the least.
No Sell: The Rachni queen catches and redirects an alpha strike from the entire squad - straight up, because she doesn't want to fight
The Ogres' armor lets them shrug off pretty much every energy weapon in XCOM's arsenal. The first encounter even has them shrug off a direct hit from the squad's Heavy weapon Platform, which fires a shot capable of hurting a frigate.
When the Citadel Forces pull out their most powerful new weapons, specifically designed to be used against the forces of humanity and other extremely advanced foes, they do rather severe damage against the Geth forces attacking the Citadel... but next to nothing to Sovereign, besides flicker his Kinetic Barriers slightly.
One Steve Limit: Joker tries to figure out a new name for Rex once Wrex joins the team much to both Shepard's and Rex's dismay. Shepard solves the problem by referring to Wrex as Urdnot.
Wrex is canonically the most dangerous organic in the galaxy. More combat experience than the rest of the cast combined, biotics, mammoth physical strength, and a really big gun. Word of God has put him at comparable to 2 platoons of Geth Assault troopers in combat.
Shepard has a similar reputation, and Wrex certainly considers him worthy of respect, but he lacks the experience that makes Wrex so dangerous. He also can't sustain his psionic hax for nearly as long as Wrex can keep up his biotic hax.
Pint-Sized Powerhouse: The Normandy here has a space-grade Blaster Launcher with the yield of a dreadnought main gunnote Shot-for-shot, the energy transfer is very similar, although the dreadnought's spinal mass driver can fire about nine more times while the Normandy reloads.. Per Word of God, the Normandy has a decent shot of taking out SOVEREIGN with a surprise attack, but no one in-universe actually knows that.
Possession Implies Mastery: Averted. The reason why humanity hasn't advanced too much from what they had at the end of the Ethereal War was because they spent the past century-plus having to learn the base principles of their tech rather than just randomly slapping things onto blackboxes like they did back then. On the other hand, they've taken the tech they did gather from the Ethereals and ran with it.
Pretentious Latin Motto: XCOM, as in Enemy Unknown, has "Vigil Confido." Cerberus appears to have "Terra Victoria."
Properly Paranoid: Played with. XCOM is far more suspicious and aggressive than the System's Alliance, and their basic gameplan involves the concept that every conflict they fight in will determine the fate of the human species. Because of this, they don't hold back one iota and absolutely adore shock tactics and rampant abuse of every trick they have. This has somewhat raised tensions with the Council races however.
By extension, the Coalition doesn't consider the Council anywhere near paranoid enough. Since the Council has basically gone all "Ah, yes, the Ethereals' 'Great Enemy.' We have dismissed this claim," XCOM has started playing brinksmanship games with them. A Council motivated by an aggressive-sounding XCOM into increasing and improving its military will make a much more useful ally whenever the Great Enemy actually shows up.
Quit Your Whining: After Wrex talks about the Krogan giving up on life and not caring enough to cure the genophage.
Shepard: "Get the fuck over yourself. You want to save your people? Stop bitching and actually do something about it. Demanding pity for being too pathetic to fight for your own survival isn’t going to fix anything!"
Rage Breaking Point: Tevos of all people loses it when Udina informs the Council that humanity will be going on the hunt for Saren.
Reality Is Unrealistic: Various readers complained that it was unrealistic for humanity to still be so xenophobic so many decades after the Ethereal War, forgetting that mankind is really able to hold long-lived grudges until they die and get stuffed. Crusades and slavery, anyone?
Red Baron: Shepard has been dubbed "the Carnifex of Kar'Shan" after he killed the Batarian Hegemony's chancellor. By telekinetically grabbing his ship out of the air and throwing it into the capital building. Shepard hates the title and nearly loses control of his psionic powers when a newbie calls him that because a bunch of civilians were killed in the crossfire.
The rachni that were responsible for the Rachni wars were a a small part of their own society that escaped the Ethereals, and encountered the Citadel by accident.
Shadow Archetype: Possibly the Thorian to the Rachni Queen. Both use are proficient in Mind Control, but the Thorain uses its powers to dominate others while the Queen primarily and apologetically uses her powers out of necessity. The Thorian is quick to resort to violence, but the Queen is open to negotiation once it becomes an option.
On Ilos, Shepard psionically yanks a Geth Prime that took a potshot at him out of a smokescreen, yelling "Get over here!"
The commander of the Citadel Defense Fleet is a turian named Rufus Scipio. There's also a turian applicant named Maximus Antillar. Both are characters from the Codex Alera.
The Stations of the Canon: One complaint about the story is that it follows the original game's plotline fairly closely, at least once Shepard's aspect of the story begins. On the other hand, there is a significant amount of deviation from the original game's plotline and an exploration of how the Ethereals' actions impacted the setting's history.
The author has stated flat-out that the timeline will change significantly after the events of Mass Effect 1.
“I can’t shake the feeling we’re gonna walk in there and find this guy petting a white cat and saying he expects us to die.” Erin rolled her eyes. “Shut it, Sam,” she said. This isn’t a movie, that’s not going to happen.”
Troll: Jaroll the Salarian Councillor during the Second Contact Incident. There's a reason that Councillor Tevos had a countdown timer on her omnitool labelled "Days Until Jaroll Retires".
“A toast then, to the dead. We honor their memories and the things for which they bled. A toast to those gone, but remembered well. May they be ready to catch us, on our fall to hell.”
Tragic Monster: The chryssalids are mutated and intensively-modified rachni.
Tranquil Fury: Nilhus's attitude after learning of Binary Helix being responsible for bringing back the Rachni.
Wrex is the only Krogan to control the Blood Rage.
The Unfettered: XCOM in general, They will stop at nothing to protect Humanity, including dropping Chryssalids onto a batarian city that was selling human slaves captured on Mindoir. If their actions make you feel uncomfortable about who to root for... it's entirely intentional.
Jack Harper counts; he manipulated the son of a high ranking XCOM captain to assist him in getting the batarians to attack the colony of Mindoir so as to prepare humanity for war.
Unskilled, but Strong: For all their raw power, the Ogres have no finesse, which allows Wrex to beat them by fighting smart.
Unstoppable Rage: The thing that sets Wrex apart from other krogan? He's not the strongest krogan. He's the one who can maintain control of the krogan Blood Rage to make it even deadlier than it normally is.
XCOM's standard tactics. If you attack them, then be ready to run like hell. They reacted to a slaving raid on Mindoir by dropping living Chryssalids (thankfully sterile) on the world that the slaves were taken to. In the next fifteen years, they proceed to shatter the Hegemony and decapitate its leadership. The humans then decide to attack Rannoch after the geth attack them, launching a complete and total assault against the main geth fleet with the intention to exterminate their species utterly.
Up to Eleven: The Thorian. It is now a fully psionic creature that had been tortured by the Ethereals, and it is pissed at humanity.