Fan Fic: Chaos Theory

But mostly? The assorted plans at play here would be going very, very wrong due to the actions of a no-name, no-count, utterly talentless Magus by the name of Shirou Emiya. He had no magic worth mentioning, no combat experience of note, and no plan for or knowledge of the War he was about to enter. He did, however, have one trait that had derailed a countless number of such grand, far-reaching schemes throughout history.

You see, he really, really wanted to be a hero.
Chaos Theory, the final lines of the story's Prologue

Chaos Theory is a Fan Fic in the Fate/stay night branch of the Nasuverse. Posted on Fanfiction.Net by the Moczo siblings, who wrote Manchester Lost, Uninvited Guests and Infinity, among other assorted fics, the plot of the fic deals with the ripple effect of how one seemingly insignificant choice can completely and unrecognisably rewrite fate.

In this story, the proverbial "nail" in question comes when Shirou Emiya first meets Ilyasviel von Einzbern, who in Canon delivers her warning to him about summoning a Servant before departing. Here, however:

Shirou stopped, and watched the girl stroll past. She… something about her just said 'do not follow me' to his mind. Like he was risking his life by approaching her. But…

Sometimes, it's the tiniest little things that can change the world.

Shirou sprinted after the departing girl, shouting, "Wait, wait up a second!"

The girl, her eyes wide with more shock than should probably have been in them, turned to face him. "Um... huh?" She said, eloquently.

Shirou knelt down so that he could look her in the eyes, and put on his warmest smile. Her clothes looked very well made, so best to assume she had money and behave with proper respect; though really, he would have anyway. Even if she wasn't actually upper-class, it was the rare little girl who wouldn't get a small thrill out of being talked to like she was a Lady. "Hello, ojou-chan. Are you all alone?"

Having believed Shirou to be—like Kiritsugu Emiya—a heartless person, this act of kindness throws Ilya off her game. In addition to that, the effects of this meeting, though small at first, steadily propel the Holy Grail War into a never-before-seen direction, where friends might become foes, the people you trust are keeping secrets from you, and villains with far-reaching schemes find their plans derailed—and in some cases, enhanced—by simple chance.

In the middle of all this chaos are two souls fighting for survival. Caught in a conflict he never saw coming (and this time without Rin Tohsaka as his guide and ally), Shirou must, along with Saber, find some purchase in a conflict that could destroy everything he cares about. Meanwhile Ilya, who grows closer to Shirou than she ever thought possible, must face her past and her inner demons as she fights a War that will not only test her most fundamental beliefs, but just might prove to be her much-needed salvation.

This story, considered by some fans to be a Fanon interpretation of the never-made "Ilya Route", takes many elements from the Heaven's Feel route while nonetheless telling its own unique tale. Started in 2010, it combines the Moczo-brand style of humour (Uninvited Guests & Manchester Lost) with their other strengths of superb plotting, characterisation and story-telling (Infinity), to create a worthwhile reading experience.

This story comes with very positive recommendations on TV Tropes.

Chaos Theory contains examples of:

  • Affably Evil: Caster Alter and, later on, Lancer Alter.
  • An Arm and a Leg: Gilgamesh loses his left hand in the battle against Saber, Berserker, and Archer. Later, Lancer Alter cuts off Saber's legs to stop her from being consumed by the Shadow.
  • Anti-Hero: Ilya shifts into this at the start of the story—much to her consternation, because Being Good Sucks due to coming with further moral boundaries that she's forced to adhere to for Shirou's sake. The later perks, however, make her less disgruntled about it.
  • Arson, Murder, and Jaywalking: Upon learning that Shirou left the house while sick and hasn't been back for several hours, Fujimura is worried that he "could be passed out in a ditch and die of illness! He could be murdered! He could get me fired."
  • Big Bad Ensemble: It's a combination of Kirei Kotomine, Gilgamesh, Zouken Matou & Caster Alter. The Dark Grail (and its creators) serves as the Bigger Bad, as in canon
  • Because You Were Nice to Me: This is the fic's turning point from canon: the whole reason Ilya grows fond of Shirou.
    Ilya: Because he was nice to me. He was sweet and kind and treated me like I was special when he didn't have any reason to, and he never asked for anything in return. You'd be surprised how rare that is.
  • Being Good Sucks: Ilya becomes frustrated with how much harder it is to do right rather than easy, to the point of a near-tantrum over it. It pays off some in Chapter 15, but even afterwards she's still quite frustrated.
  • Big Sister Instinct:
    • If you don't want Ilya to sic Berserker on you, then do not harm a single hair on Shirou's head. Rider finds this out the hard way.
    • As much as Rin might not want to admit it, she has Sakura's back.
    • In Chapter 22, Shirou goes mental on Caster for hurting—and nearly killing—Ilya after taking her Command Seals.
  • Blade Brake: Lancer Alter does this while fighting Berserker. Saber does it soon after while fighting Gilgamesh.
  • Blood Knight: Lancer Alter's enthusiasm for a good fight makes his uncorrupted self seem almost calm.
  • Casual Danger Dialog: Rin and Archer banter some while dodging a rain of fire attacks.
  • Cerebus Syndrome: The first couple of Chapters are pretty light-hearted, but there's some dark moments to be found in the story, and increasingly so. It never loses it's humor and heartwarming, though.
  • Character Death: The major characters that have fallen so far, in order: Fake Assassin, Kuzuki, True Assassin (apparently), and Berserker.
  • The Chew Toy:
    • Shinji, arguably even more than in Canon.
    • Lancer still can't catch a break, what with his being eaten by the Shadow and turned Alter. Though, the fic does Throw the Dog a Bone several times.
    • Berserker will always die before the route even reaches the climax, and this route is no exception, with him falling under Caster's command thanks to Rule Breaker, and Saber being forced to destroy him in order to save Shirou, Ilya, Rin, and Archer.
  • Clueless Chick Magnet: Shirou, as usual. He starts out with Sakura, then picks up Illya and Rin, and then Saber. Was there any other conclusion?
  • The Corruption: The Shadow does this to Caster and Lancer, rather than devouring them outright as was planned by Zouken. It begins slowly taking over Saber, as well, after her contact with it.
  • Cut His Heart Out with a Spoon: Gilgamesh promises to really torture Archer in this kind of manner after seeing the latter make copies of his Noble Phantasms.
    Gilgamesh: You know who I am, don't you? Then you know what I can do. Take this as the word of Gilgamesh. I will obliterate you, Faker, but not before I make you suffer beyond your imagination. I'll hunt you down wherever you hide, and drag you kicking and screaming into Hell. First I'll hack off all your limbs, cut off your eyelids and force you to watch as I violate your pretty master in every way imaginable before gutting her like a fish and force-feeding you her entrails. Then I'll peel your flesh off in strips, slice apart every muscle, carve my name into your bones. And that is only the opening act. When I am done, your name will go down in history as a synonym for unspeakable torment, a fitting example of what happens when the king is challenged.
  • Dark Action Girl: Rider. Also, arguably the course for Caster Alter.
  • Defrosting Ice Queen: Saber and Ilya, in different ways.
  • The Dragon: True Assassin to Zouken Matou, being more powerful than him overall but loyal enough to not backstab him.
  • Dragon with an Agenda: Caster Alter is both, to different characters:
    • The former to the Dark Grail, which hasn't enough intelligence beyond its loneliness and hunger.
    • The latter to Zouken Matou, who thinks she is incorrigible but will serve him until he no longer needs her; in reality, she's the one pulling his strings.
  • Drunk On The Darkside: Servants corrupted by the Shadow most definitely get a kick off the near-unlimited prana it provides. Even Saber isn't immune to its lure.
  • Dude, She's Like, in a Coma!: Ilya does this with Shirou to help him with his mana deprivation caused by Rider. Multiple times.
  • Enemy Mine:
    • Notably averted between Shirou and Rin: because Ilya chooses not to attack, the Emiya-Tohsaka alliance doesn't form on the first night and they become rivals.
    • Between Ilya and Saber since Chapter 16.
    • An Emiya-Einzbern-Tohsaka alliance is officially established in Chapter 24.
  • Even Evil Has Standards:
    • Kirei has an internal monologue about this, comparing himself to Zouken:
      But at the very least, even on those rare occasions that his conscience (yes, despite what anyone else might say, he did have one) flared up, he could always say, "At least I'm not Zouken." The elderly Magus disgusted him, repelled him in every way and on every level. Kirei was, after all, completely selfless in his own way. His means of doing so was bizarre, perhaps, but he cared only for other people.
    • More conventionally, Gilgamesh reacts this way to Caster Alter (and her "pet" shadow monster, Lancer Alter) in Chapter 11.
    • Ironically, though not a case of Evil having standards, Ilyasviel mentally compares Gilgamesh's attitude to that of the Shadow in Chapter 15. Gilgamesh is found... wanting, due to his conceit and smug-superiority coming off as more contemptable than the Shadow's anguished loneliness.
    • Caster Alter feels the same way about Zouken as Kirei does. She may be evil and steal people's souls, but at least she's not so slimy about it.
  • Evil Versus Evil: Kirei Kotomine, Zouken Makiri, and Caster Alter all have conflicting plans and are therefore against each other.
    • And all the while, Lancer Alter just wants to fight against the greatest warriors of history and myth, and he's perfectly willing to damage the plans of people like Zouken and Caster Alter, even though the latter is supposed to be controlling him and the former is supposed to be controlling the latter.
  • Exploited Immunity: Caster Alter, while controlling Berserker, uses explosive attacks that won't harm him but will hurt his opponent.
  • Face Death with Dignity: Herakles/Hercules does this.
  • Fastball Special:
    • Hilariously, Shirou does this to Shinji in Chapter 4, by throwing him into Rin.
    • Then, Archer throws Rin into a thorny bush in Chapter 10.
    • And then Saber does the same to Shirou in Chapter 13.
    • In chapter 14 Rin gets chucked by Archer (again) into a pond. At least it's better than a bush.
  • Five-Bad Band: One gets formed partway through the story:
    • Big Bad - Zouken - the Grail serves as the Bigger Bad
    • Co-Dragons/Evil Genii - Assasin & Caster, both being cunning and both being second-in-command to Zouken
    • The Brute - Lancer - only in it for the fight and not the most powerful
    • Dark Chick - Sakura - not a willing participant and more sympathetic, but crucial
    • Team Pet - Shinji - being mostly rather useless compared to the Mages and Servants
  • Five-Man Band: The good guys are steadily teaming up, and as of Chapter 23 the structure seems to be as follows:
    • The Leader - Rin Tohsaka - the one who formulates the structure of the team and coordinates the others as a whole
    • The Lancer - Ilyasviel von Einsbern - being that she bickers and clashes with Rin's personality
    • Smart Guy - Archer - wisecracking, but also the most experienced of the group and an important strategist
    • Big Guy - Saber - with Berserker's death she's the most powerful physically, and her partial corruption puts her control in doubt
    • The Heart - Shirou - the weakest overall, but he's the glue holding them all together and the most "human" of the group
    • Sixth Ranger - Rider - forcibly taken by Rin from Shinji, but aids them because it suits her agenda for now
  • Flat "What.":
    • The response of Archer, Lancer, and True Assassin to Caster Alter in Chapter 9:
      Lancer: What...
      True Assassin: The...
      Archer: [thinking] ...Hell?
    • Shirou responds this way when Ilya goes from near-crying to cheerfully calling him adorably shy.
  • For Want of a Nail: The nail in question? Shirou responds to Ilya in their first meeting. The story snowballs from that point.
  • Freak Out: Archer goes into borderline hysterics when he realises that on the Holy Grail War's first night Ilya and Berserker didn't launch their first attack. Rin does the same upon seeing the Shadow for the first time. Shirou freaks out when he learns Ilya's actually nineteen and isn't as innocent as he thought she was.
    • Rin has a two-tiered Freak Out in Chapter 6. The first is when Berserker arrives, wipes the floor with Rider, and then turns his attention on Rin. The only reason she even lives is because Shirou delays Berserker long enough for Archer to intervene. The second comes from Rin's misreading of Berserker's refusal to attack Shirou: she thinks that Shirou has a secret alliance with Berserker's Master, and the prospect of needing to defeat a tag-team of Saber and Berserker for a shot at the Grail seems impossible.
  • Friendly Enemy: Rider. Even when she takes some of Shirou's prana to ensure Sakura doesn't die, she's very considerate about it.
  • From Bad to Worse: Caster takes Ilya's command seals with Rule Breaker. Now she has BERSERKER under her control.
  • Heel Realisation: Ilya's is a slow-burning one, but it's coming.
  • Hero Antagonist: Rin, as Shirou's opponent during this War. However, they come to an agreement due to the Bigger Bad of The Shadow and the Dark Grail.
  • Hypercompetent Sidekick: Archer seriously gives off this vibe at times, since almost every significant win for the good guys is at least partially due to something he did.
  • In Spite of a Nail: The narrative uses "Heaven's Feel" as a base for the most part - most likely due to Sakura and Ilya's greater prominence in that timeline - so despite many differences and some expanded character roles compared to HF, the storyline plays out like HF with a Spanner in the Works (though some elements are just FSN routes in general): Shirou has some measure of attraction for Sakura; Zouken Matou decides to participate in the 5th War which leads to True Assassin consuming Fake Assassin; Lancer still has a string of really bad luck, though after his death he becomes much more competent and independent as Lancer Alter; Berserker is still The Dreaded and a Game Breaker in the early stages of the story and he dies an antagonist, to show how dangerous the real villains are and because he's too powerful, as well as being under the control of the Dark Grail (in a manner of speaking). Pheeeeewww...
  • Knight of Cerebus: Caster Alter.
  • Ladies and Germs: Hilariously invoked by Archer in Chapter 3 (albeit non-verbally) when preparing to kill Shirou, Shinji, and their Servants:
    Archer: Good-bye, Ladies and Gentlemen. Well... ladies, gentleman, and Shinji. I am the bone of my sword.
  • Locked Out of the Loop: Shirou still has no idea that the sweet little ojou he met in Chapter 1 is a participant in the Holy Grail War. She slips up in Chapter 21, where it looks like it will finally come out... Then From Bad to Worse. Chapter 23 has Rin finally explain the pieces of the puzzle to a befuddled Shirou.
  • Mistaken for Pedophile: Shirou in Chapter 23, much to his chagrin. Early on, he was only worried about looking like the time he spent with Ilya was sketchy, but upon finding out that she's actually nineteen years old...
    Shirou: It means all those innocent things she did! Silly kid stuff! They weren't innocent, Tohsaka! They were... they were... she was... those were dates!
  • Morality Pet: Shirou becomes one for Ilya, much to her chagrin.
  • No Social Skills: Ilya is clever, but her isolation and lack of any real friends throughout her life means that she's not very graceful in certain social elements; in Chapter 23, she seems to assume that Shirou knew he was going on dates with her despite thinking she was ten, and that even if he didn't realise it, it was perfectly acceptable. Whichever one it was, Saber and Rin ultimately get the wrong impression.
  • Noodle Implements: In Chapter 1 (not counting the prologue), Ilya and Rin vaguely contemplate expressing their anger at the one perceived to be responsible using "power tools" and "a variety of bladed instruments," respectively.
  • Not What It Looks Like: In Chapter 23. When Shirou freaks out about the fact that he was, essentially, going out on dates with Ilya, Rin and Saber mistakenly assume that he knew they were dates and that he's freaking out because he didn't know she was nineteen. Of course, Shirou's inability to connect with people and empathise properly meant that he didn't even realise that Ilya considered them dates - he thought that he was in an Intergenerational Friendship and misread Ilya's intentions, which then leads to her digging him in even deeper in an attempt to save him. A long, heated discussion ensues offscreen for twenty-four minutes before the matter is resolved.
  • Or Was It a Dream?: Caster's attempts to corrupt Sakura via illusions projected into her sleeping mind.
  • Oh, Crap:
    • After the Freak Out mentioned above, Archer quickly realises that the War he's currently in is not the same one he'd originally dealt with when he was alive.
    • Anyone during their first encounter with a Shadow-corrupted Servant. Even Zouken!
  • The Ojou: Before he learns her name, this is how Shirou addresses Ilya.
  • Precision F-Strike:
    • Lancer's is Cut Short when Caster Alter deploys the Shadow to attack him in earnest.
    • Archer gives one when he sees Rin, Shirou, and Ilya are all down and he has to try to fight Berserker without hurting them further.
  • Protectorate: Shirou is this for Ilya, as of Chapter 4.
  • "The Reason You Suck" Speech: Shirou (of all people) gives one to Shinji in Chapter 4.
  • Revenge: After Kuzuki's death, this becomes Caster's motive.
  • Shout-Out: Sakura's first dream sequence is a HUGE one to Baldur's Gate II.
  • Slasher Smile: Gilgamesh sports one of these in Chapter 25. It is unnerving enough to scare Kirei.
    Gilgamesh turned to face him, and Kirei felt a slight twinge of fear for the second time. Gilgamesh was smiling. Widely. Ear to ear, an expression of almost childish happiness, but something in his eyes...
    Gilgamesh: It seems I smile when I'm truly angry.
  • Superpowered Evil Side: Caster first, with Lancer following on from that.
  • Sword Plant: Saber tends to stab the ground and use her sword for support when she's exhausted. Archer stabs a projected Kanshou into the ground while running as part of a strategy.
  • Take That, Scrappy!: Invoked by the author in regards to Shinji, whom he admits to despising—apparently, making him suffer is a joy for him. Not that anyone really minds.
  • Teeth-Clenched Teamwork: In Chapter 10, between Archer, Lancer and True Assassin.
  • Tempting Fate: Caster Alter declares that there will be nome more last-minute rescues to stop her from killing Shirou. Immediately after she says so, Rider breaks down the front wall of the room they're in. Zouken hangs a lampshade.
  • This Is Gonna Suck: Lancer says it, word-for-word, in Chapter 10.
  • Tranquil Fury: Archer manages to anger Gilgamesh to the extent that the latter goes from roaring in rage to speaking softly. While promising to inflict untold torments on Archer.
  • Undying Loyalty: Rider to Sakura, as per canon.
  • Wham Episode:
    • Chapter 8.
    • Chapter 21 tops it. Caster Alter now has control over Berserker after using Rule Breaker on Ilya, Saber is missing her legs, and Shirou had a hole blown through his chest.
    • Chapter 22: Rider nearly dies fighting Berserker, she teams up with Assassin and Lancer Alter to stop him... and Lancer Alter kills Assassin in revenge for an earlier slight. Shirou draws on incredible power to protect the now-defenseless Ilya from Caster Alter and injures her. Saber is able to pull beyond her reserves and destroy Berserker to protect Shirou, Ilya and Rin... but only because she somehow drew on some of the Tainted Grail's power. And Caster Alter is smiling at this last bit.
  • "World of Cardboard" Speech:
    • Ilya gives an internal monologue to this effect in Chapter 4, regarding Shirou.
    • Also, Lancer Alter in Chapter 13.
      Fallen? Saber... you know who I am. I spent my entire life desiring battle. Glory. And I spent each and every bit of it as someone's damn lapdog. The Hound of Chulann... but oooooh, things are different now! My new Master runs only on instinct. It has no goals, it's just a monster that wants to feed, to hunt and kill the Seven Servants. To face down each one, defeat them, and devour them. That's all it wants to do, and all it wants me to do. And I can do it however I want, with my so-called Master too mindless to do anything about it but follow me in a frenzy, begging for scraps. I don't have anyone holding me back, Saber. Nobody's leash stopping me from claiming the ultimate battle that I always wanted.
      Fallen? For the first time in my life,
      I'm free!