Author's note before we begin: I wrote this thing on a whim after realizing there is no Matou Shirou fanfiction so yeah, it's gonna be a bit random till I figure out the story until the Grail War.
On a side note, since we have no clue on the Matou magecraft other than bugs and absorption, I'm gonna have to make my own.
Onto the story.
"Gluttonous King, Lord of the Flies," Shirou silently chanted, forcing prana to move out of his body and into one of his grandfather's familiars.
His magic circuits, 27 of high quality if his Grandfather was to be believed.
Every time he opened them gave him the impression of being stabbed by 27 knives all at once.
More than that though, was the faint memory that always accompanied the opening, that of a cursed mud flowing into him.
Despite his closed eyes, he could feel the gaze of his master and could smell the fluttering familiars around him.
The worm struggled, but in the end, he could feel it. A strand of consciousness appeared between him and the worm, and he could feel that it would do as it was ordered.
Despite its relatively small size, Shirou instinctively knew that the more prana poured into the worm, the greater it would become. Perhaps it was because the worms had partaken of his flesh and he of their lifeforce, but this whole thing was strangely easy for him.
The worm scurried up into an offered hand, Shirou was tempted to add prana to see what would happen.
Would it mutate into a Blood Worm like the ones that made up his grandfather's body? A Blade-wing Worm? Or maybe something different entirely.
Shirou didn't get to finish pondering on his thoughts as the sound of clapping caught his attention.
"Congratulations Shirou." His grandfather said, a shell of a smile on his decrepit face. "It seems that your Magecraft studies are going well."
Shirou nodded and bowed a bit, "Thank you grandfather, but this is only the beginning." He stated, "I'll need more if I am to survive this world."
Zouken smiled, it was a grotesque smile, one that would make any child or adult cry. "Your thoughts are correct Shirou, but for now, your body cannot handle the strain of commanding even one of my familiars."
The thousands of bugs in a fake body looked at his grandson's twitching hand, struggling with writhing blood.
"It seems you've flooded your hand with too much prana," The blood vessels started popping as the old monster told Shirou.
"Oh," The child made a small noise, looking at the injuries with curious eyes. "This body is made of curses," He chanted, twenty seven circuits opened and flooded the injured hand with prana.
You'd think that would just destroy the hand further, fortunately Magecraft was weird and flooding the body with prana with the thought of using a Sorcery trait was different from flooding the body with prana with the thought of trying to separate it from its home.
The writhing blood vessels calmed down as the skin stitched itself back together.
"Hoh, it seems you've gotten better with your Sorcery Trait," The Matou Patriarch complimented his student.
Shirou shook his head, "It was all thanks to you grandfather," How Shirou could say that without an accusatory tone not even Zouken knew.
"Hmm," The old monster hummed, taking control of his familiar back with but a thought. Shirou flinched at having the connection so suddenly broken, but calmed down soon after, panicking wouldn't help him. "Go study for the rest of the day, we have something of a visitor coming home."
Shirou obediently nodded, watching the worm jump out of his hand with a clinical eye. "Yes grandfather,"
He trudged off to the family library with a new thought in his head.
'How would I stop grandfather from severing the connection?'
It was a simple thought, but one that would no doubt impact his future.
It was when Shirou was writing notes as he researched a curse he found interesting that he met the newest occupier of the mansion.
A blue haired boy who glared at him like he was the very incarnation of evil.
Which he-
"We shall inscribe upon you all the evils of the world!"
"O' Evil God, take this and be appeased!"
"Incarnation of Human sin! Ḁ̵̳̤͌̃̀͒n̸̛͔̜̆͗̄ǵ̷̢̯̄̏̎ͅr̵̟͈̬̯͐̓̈́̈́a̵̢̫͍͒̑̂͘ ̷̛͈̏M̴̡̬͆͝ả̵̦̼̿̈́͠i̸̛͖̥͇͕n̴̻͕̑̂̋͂y̵̼͔̚͘ứ̶̫̝̤͋!"
-wasn't… probably.
The boy disappeared behind the door, but not before giving him a final glare.
Shirou was confused, was that the new houseguest? Or was he another of his 'Cousins'? He did look a bit like that drunkard who wandered the halls and visited the purple haired girl's room.
Shirou put the thought off his mind, it wasn't his problem.
Unless they made it his problem.
In which case he could finally try out some of these curses in these many, many books on curses.
Really, had he not known his grandfather's main attacks were using familiars, he would have thought they were curses.
For instance: One spell detailed a curse which turned all the fat in one's body into fire, or another in which a spell turned the opposing Magus into stone utilizing their own prana.
Another still projected fear into the opponent's hearts whereas another transferred an explosion curse through touch.
Hmm, maybe he could turn a familiar into an explosive?
A Blade-wing worm that exploded would certainly be lethal…
Thoughts for later when he could actually control the worms without having his body explode in a shower of blood.
Even he wouldn't survive that…
Probably.
Either Way, he would need more practice in both making curses and creating familiars to ake that happen.
Though, perhaps he could use other bug themed familiars than just worms. A Centipede sword sounded pretty cool to him that even his dampened emotions gave him something.
Whether that was fear or excitement Shirou didn't know, what he did know was that these spells were all demented and horrifying.
Pretty cool though.
It was at a rare dinner where his grandfather attended that he met the blue haired boy a second time, still glaring at him like he had stolen his hard earned promotion.
At the head of the table was his grandfather, smiling maliciously ast what Shirou could only figure to be the hatred coming from the blue haired boy. To his right was where he sat, across from him the drunkard and next to him was the glaring boy while the purple haired girl sat next to him.
On their plate was a western dish, filet mignon steak or whatever.
Shirou had no idea who cooked this, let alone if it even tasted good. He had to cook for himself since his grandfather sure as hell wasn't going to.
He figured the dull eyed girl was getting nutrients from the worms since he never saw her at dinner time and was pretty sure the only nutrients the drunkard was getting was the dirt where he collapsed in a drunken stupor.
So who cooked this meal or why was a mystery to him.
Was it the new boy? Could the drunkard or the girl cook? Or was this just some massive prank from the old monster?
Shirou had no idea.
Nor did he know why everyone was looking at him with horrified eyes, bar the girl who gave him a curious look and his grandfather who looked like the cat who ate the canary and married his widowed wife.
What he did know was that this dish was actually pretty good.
Though the beef tasted a bit like pork…
But it was good food, so he continued eating it even if no one else was eating.
At the very least the blue haired kid wasn't glaring at him. He was now staring horrified as if had just defiled his mothers grave.
"Shirou," He was broken out of his ramblings by his grandfather, "I would like you to meet your cousin, Matou Shinji."
The boy gave a grudging bow, "It's nice to meet you, please take care of me." It seemed he was playing nice while grandfather was in the room.
"Shinji, meet my heir," The old worm's smile stretched as he could feel the enmity rise from the circuitless boy. "Makiri Shirou, your new cousin and future head of this house."
Shirou could hear the boy's teeth grinding together, and looked to be having an aneurysm.
Not that he really cared about that.
If Shinji was nice he would be nice in turn, if Shinji was being difficult he would be difficult in turn, if Shinji opposed him… well, new material was always nice.
For now, Shirou would try to see where this all leads. "Nice to meet you, I hope we can become good friends." He put up the smile grandfather told him to use when meeting new people.
Though by the way the boy and the drunkard flinched, he felt as if his grandfather was using him as a practical joke again…
'Oh well,' Shirou decided not to think about it anymore, continuing on to eat the pork flavored beef.
Or maybe it was pork that looked like beef?
But his grandfather told them it was filet mignon… could pork become filet mignon?
Questions for later.
It had been one year since the great fire which killed a hundred people.
More importantly it had been a year since he was adopted into the Matou household as well as nearly ten months since Shinji joined the family.
Speaking of Shinji, the boy still couldn't utter more than a few words in his presence. Shirou didn't know why and every time he brought it up to his grandfather he always laughed and told him it was something he would understand later.
For some reason he doubted that.
On another note, Shirou could now command just under a dozen of the Crest worms and was now trying to figure out how to make familiars other than worms.
Sure, despite being low tiered they were powerful, but he had a strange sense of curiosity that wouldn't stop until it was fulfilled.
Whether that be putting his hand into a fire, figuring out how spiders make their webs, or now, how to make a centipede blade he thought of so long ago.
At his request, several orange-legged centipedes were brought in, each of them venomous enough to cause swelling and mild pain to a human.
He was now in the process of mutating them into becoming large enough to cover his hand.
Which was a bit difficult when he was considering how to make their carapaces stronger as well as growing swords on their body.
He was taking cues from the Blade-wing worms but now he was thinking of maybe only trying to do one thing at a time.
Shirou sighed, feeding a mouse to his new pets/familiars.
The screams and the frantic running of the mice as they were devoured alive by the insects never failed to bring a smile on his face.
Fortunately he had at least managed to make them a bit bigger, but now they were only a few inches longer instead of the height and width he required.
Perhaps he should feed them more of his prana…
"Onii-sama," He was brought out of his ponderings by Sakura, the purple haired girl.
"Yes imouto?" He turned towards her, a question in his dull golden eyes.
She never came to his room for some reason, though for some reason Byakuya- the drunkard- frequented her room.
Shirou could guess, if only because he sensed the crest worms always feeling satisfied afterwards and he only knew of one way or that to happen. But it didn't really matter much to him.
This was to her advantage afterall.
"Grandfather wanted to talk to you about something," She said primly and properly, bowing out of the room soon after.
Ever since his new title was revealed, it seemed that she had taken a background role and didn't seem to be spending as much time in the pits as he did anymore.
Though he sometimes saw Shinji in her place though.
He wondered why, but not enough to actually ask his grandfather.
Perhaps he would learn when he was older, though for some strange reason he also doubted that.
Shaking his head of those thoughts, he released another rat into the cage containing his experiments, smiling as the centipedes devoured it.
Walking off, he went off to see what his grandfather wanted.
Smiling slightly as he heard a yelp of pain behind him.
It seemed someone tried to sneak into his room without his knowledge or permission and had been done in by his wards.
Closing his left eye, he connected it to one of the centipedes in his glass box.
Flinching slightly as the dysmorphia hit him and he realized how absolutely giant centipedes were even if there were only minimal mutations.
Also how every part of the centipede itched, almost like it was morphing or transforming into something else, something… more.
So distracted was he by the new revelation, that he barely saw a speck of blue hair and the figure of a small girl dragging him out of his room.
Shirou smirked, this meant this was all some part of a plan by Shinji and he had corralled Sakura into it.
He didn't blame them, these were all family shenanigans afterall.
Though that meant the order to visit his grandfather was false. Well, either that or this was real and Shinji just decided to take the chance while he could.
"Hmm," Shirou hummed, "Oh well," He continued on his way to the pit room, where Zouken usually laid.
Either Sakura was telling the truth, or he threw Shinji and Sakura under the bus.
Either was fine, though he felt a sense of dark delight at seeing his new family's despair and anguish.
"Damn it!" A certain blue haired boy stomped on the ground, tearing up as gnashed his teeth together. "First- first he takes what's mine even though he's not even family and now he humiliates me!"
"Ah, nii-san, you shouldn't be mad." The toneless voice of Sakura shook him from his thoughts.
"Don't tell me-" He stopped himself and took a deep breath. Wincing at the slightly concerned face on his little sister.
Sakura had been the only one to support him when he had gotten back and a ginger bastard had taken his spot as heir. Even his father, the man who he always loved, had abandoned him in favor of scurrying beneath his feet.
It was humiliating, it was a horribly shitty thing to realize even the closest to you had betrayed you.
Thankfully, even if they hadn't stuck with him, Sakura had.
"Sorry, sorry, Sakura, my anger got the best of me… ha ha." He tried to laugh it off, but it was a little weak even for him.
The dull eyed girl nodded.
"Ah, speaking of, do you know what-" Shinji was interrupted by the door opening, he felt his stomach form a pit as dread creeped into his heart.
The smell of alcohol seeped through the air, threatening to dizzy him.
The harsh footsteps of a man Shinji hesitated to call his father were heard until the man himself came in. Scruffy beard and seaweed like-bluehair, there was an element of mindlessness in his eyes.
Shinji could only gulp and try to hide Sakura behind him, but it was all for naught.
He was only eight years old.
While this man was well into his thirties.
Shirou yawned, even if he could suppress his natural drowsiness with his od, it was still unhealthy for a child to go past their sleep schedules like this.
So he fed one last mouse to his pets before heading to his bed, vaguely hearing the sounds of uncle Byakuya and his new cousins.
This was their punishment for pranking him with a fake meeting with his grandfather. Shirou was mostly ambivalent about it, but he still wanted to get them back, so it was the perfect time to meet his drunk uncle in the hallways.
So it was that Shirou headed off to sleep, to the vague noise of his family in the other room…
And that's that for our technical first chapter!
To answer a few questions I already know are coming: Yes, Shirou is evil and more akin to Kotomine and Zouken and you may have problems with that.
But you have to understand the reason behind Shirou's want to save everyone was because of Kiritsugu and his dream being passed onto Shirou who took that and ran with it as far as he could because he tried to use that borrowed dream to fill in the void from the fire.
Then we have Makiri Shirou, who's even more empty and distorted due to some horrific combination of the Grail Mud, the Tainted grail, and Zouken being Zouken.
Finally the magic Shirou uses.
It was because of Avalon that Shirou became an incarnation, so no Avalon=No UBW.
Of course this means he'll be a normal Magus. If a bit more in tune with curses than normal.