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The size of a cargo truck, with black skin that seemed like rock and filled with teeth-like spikes. And it had eyes. Lots and lots of eyes.

"What's the worst that could happen?"
Unknown (Forever remembered as the biggest idiot in the Omniverse)
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Lusus Naturae, Latin for 'Freak of Nature', is a Sword Art Online Fanfic by SaintInfernalNeos.

One night, when Kirigaya Minetaka was driving home after spending New Years with his wife's, Kirigaya Midori's, parents, a thunderous 'Boom' echoed through the mountain road.

Against better judgment, they proceeded to investigate with the excuse that someone might be hurt, despite the fact that there shouldn't be anything around for kilometers.

When they came across a strange 'pod', they knew they should have walked away. But, at that moment the pod proceeded to eject a confused and weird-eyed little kid. He seemed harmless enough, and he made their daughter laugh, so really, bringing him home couldn't hurt, right? After all, what's the worst that could happen?

...as many people, like Kayaba Akihiko, would find out years later when meeting 'Kirigaya Kazuto', quite a lot of hilariously sinister things, actually.

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Kazuto also runs into many of our heroines IRL before Sword Art Online even launches, which has varying effects on each of them.

The fanfic can be found here.


Lusus Naturae provides examples of:

  • Adaptation Personality Change: Shino, due to Kirito being at the post office during the incident that marked her in Canon, and disarming the robber in a very Alien way. This Leads Shino to become obsessed with aliens and conspiracies. Kirito is this too due to his alien nature.
  • And I Must Scream: The device used by Kikuoka and his troops to contain Mirai is this, a reverse engineered Fulldive device that uses the Medicuboid and Nervegear's ability to shut down its wearer's physical sensations and motor functions to keep the imprisoned in their own body.
  • Armor-Piercing Question: Kikuoka accidentally asks one to Kirito in Chapter 19 without realizing it:"Why do you smile." The results...arent pretty...
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  • Ax-Crazy: Eiji, becomes clearly unhinged when he decides to join up with Kikuoka to get back at Kirito and Mirai.
  • Being Tortured Makes You Evil: Eiji was captured and tortured by Mirai to get information on Kirito, caving in when the latter threatens Yuna. This, combined with the revelation that Yuna was not only fully awareof, but also fully accepting and in love with Kirito, causes him to snap, and become Kikuoka's attack dog.
  • Big Damn Heroes: Subverted. Suguha and Kage's attempt to save Kirito from Wayrum's capture fails, as stated in Reality Ensues.
  • Bioaugmentation: Upon capturing Mirai, Kikuoka ad his scientists use pieces of her genetic material to turn Eiji into a hybrid Super Soldier who easily repels a group of Tryums with lethal results.
  • Bizarre Alien Biology: Kirito and every species he ever ate. The Tryums as well.
  • Blue and Orange Morality: Kirito, at first. He gained enough knowledge of human norms to eventually become a Cloud Cuckoo Lander.
  • Butt-Monkey: Waryum the Tryum.
  • Cloudcuckoolander: Kirito has...a unique thought process.
  • Dark Is Not Evil: He's weird, but Kirito is still a pretty nice dude all around.
  • Didn't Think This Through: Upon rearranging the DNA of the Sleeping Knights to save them, the others point out that they now have no other place to go or any supplies since he snuck them out of their hospitals. Fortunately, Kirito manages to arrange for Kayaba to use his newfound wealth to cover their needs and supplies.
  • Eldritch Abomination: 'X-K0940', or Kirigaya Kazuto, is a product of an Alien Experiment to 'enhance' an egg that survived being exposed to a very specific type of radiation from a bomb they dropped on its native planet, for no reason other than to win a bet.
  • Everyone Has Standards: For all his moral scrupples Kikuoka finds Eiji's bloodthirsty nature disturbing and has an Explosive Leash placed on him.
  • Exotic Eye Designs: Kirito's real eyes are described as 2 red slits with golden sclera filled with black veins.
  • Extra Eyes: In his Non-Human form, Kirito has eyes all over his body. They also have Hellish Pupils.
  • First Kiss: Between Kirito and Yuuna in chapter 6. note 
  • For Science!: Half the reason Kirito was experimented on by the Tryums.
  • Healing Factor: Kirito has a VERY potent one. Waryum's species has one as well, though not to the same absurd degrees.
  • Higher-Tech Species: The aliens known as 'The Tryums' are this.
  • Human Aliens: Kirito is this most of the time due to Voluntary Shapeshifting. As a bonus, his internal physiology is normally human due to him thinking of himself that way.
  • Humanity Is Infectious: Being raised by humans has clearly affected Kirito's personality. Waryum also admits to being affected as a result of hiding amongst humans for so long.
  • I Have Your Wife: Kikuoka attempts to cajole Kirito into by using his loved ones as leverage, threatening that with their involvement in covering him up, it would be easy to ruin their careers(in thi case of Shigemura and Midori) or future prospects(in the case of his loved ones who are still in school.
  • I Just Want to Be Normal: Downplayed, but to a degree Kirito is interested in wanting to be normal which is why he became interested in SAO in the first place.
  • Instant Expert: Thanks to his biology, Kirito was capable of singing rather decently on his first try in Chapter 1 note , which provoked Yuuna to declare him her singing rival.
  • It Can Think: Waryum is shocked by the fact that Kirito is not only capable of speech and thought, but that he can even dream and have nightmares.
  • LEGO Genetics: An ability Kirito discovers when trying to save a crow, later named Kage-chan, and later uses to save the Sleeping Knights. Also uses it to fly, shoot plasma from a head-arm, create weapons out of his bones, morph his arm into a deadly bio-blade, etc.
  • Nigh Invulnerable: Kirito is this thanks to his very potent and adaptive Healing Factor. Waryum theorizes that the only way to put him down permanently would be to toss him into the sun.
  • No Social Skills: Kirito. His thinking anime is a good reference for social situations is one of the LEAST relevant examples.
  • Omake: Starting with Chapter 5 note , each chapter is followed by a series of omakes titled 'Lusus + Vampire' which is a Crossover with Rosario + Vampire
  • Organic Technology: How Tryum technology is described as.
  • Reality Ensues: In Chapter 4 note , Suguha rode Kage the Crow through the air to mount a rescue of a captive Kirito...and failed since, a) Suguha was weak and off balance as a result of being exposed to the cold temperatures and strong winds for the first time, b) Kage was still a bird not that bright so ended up speeding straight into a cramped location, and c) Waryum was still armed and had his weapon primed. The result: Waryum shoots down Kage (his weapon was Set To Stun) and Suguha while barely getting hurt himself. SIN even refers to it as an example of this trope in the Author's Notes.
  • Really 700 Years Old: Kirito, while appearing the same age as his canon counterpart is in fact over 350 Earth years old according to Waryum.
  • Shout-Out: When going to Akibahara for a gift for Suguha, Kirito decides on a figurine of a red haired swordsman with a cross-shaped scar on his cheek.
  • Stock Quotes: Some of the curious/sinister/hilarious Scene Break Quotes are these. Some are Tropes and others are just made up to fit the scene.
  • Shapeshifter Baggage: Kirito's Natural form is the size of a 16-wheeler truck but he has no problem taking the shape of a normal human.
  • Spared by the Adaptation: All the Sleeping Knights. See LEGO Genetics above.
    • And, due to SAO not being a death game, everyone who died playing SAO in canon (not that we know what they were doing). Also, Kayaba doesn't become a virtual ghost.
  • What Could Possibly Go Wrong?: What Minetaka thought after resuming the drive home. Also, the following Scene Break Quote.
  • Why Did It Have to Be Snakes?: Kirito is afraid of water. Ironic, in that it's confirmed he had been 'deployed' in fully aquatic planets before and that he should easily be able to adapt and survive even in the deepest parts of Earth's ocean.
  • You Called Me "X"; It Must Be Serious: Argo is disbelieving of Kazuto's claims of being an alien during Chapter 5. Then, when he insists it's not a joke, he refers to her by her first name, something he's never done in all the time he's known her.
  • You Can't Thwart Stage One: Inverted. Kirito stops the Death Game before it even starts. By Confronting Kayaba, and convincing him not to go through with it.

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