Fair Warning: this chapter contains Zouken Matou and all that implies.

Kariya's Legacy

Shinji could remember that night clearly as the present. The memory was never far from his mind, those final nights before the Fourth Holy Grail War. He was supposed to have left earlier, sent abroad while the secret conflict was conducted, but an expected delay in the flight had him staying in the manor another night. Unable to sleep, he had gone to the kitchen for a snack.

Things that seemed trivial and meaningless branded into perfect clarity. The quiet roar of the air conditioning, the smell of the kitchen and the cool floor seeping through his slippers. More remarkable was how loud the quiet could be. That was how he found Uncle Kariya.

"Who's there?" he demanded.

A figure shuffled around in the dark, a hood drawn over his head, shadowing his face from what little light there was. He stooped over and leaned for a better look at the boy.

"Shinji? What are you still doing up?"

Stepping back, Shinji strained his eyes to see who it was. He didn't recognize the voice. "Who are you?" he asked.

"Right, sorry. It's been a while hasn't it?" The figure gave a dry chuckle, drawing his hood back. Stepping into the moonlight, Shinji got a better look at his estranged uncle. His hair had gone white and lank, instead of its natural blue or even the black he had dyed it to. And his face sagged to the side, like he had suffered a stroke. Pale as the moon, he looked like he had seen better days since the last time he had seen any picture of him, limping along.

"Uncle Kariya?" Shinji hesitantly probed.

"Not a pretty sight, I know."

Shinji could see that his uncle was in pain standing up. "Can I get you something?"

Kariya coughed, disguising it as clearing his throat. "Water, please."

Letting the man find a seat. Shinji filled up a glass and brought it over, setting it in front of his uncle when it was clear his hands were shaking. Kariya grabbed it and greedily gulped it down.

"You're bigger than the pictures suggested," Kariya said, holding the cup in a death grip, then looking about the room. "Is Sakura around?"

"It's half past midnight," Shinji replied. "She's sleeping."

"Ah. Right…"

Shinji stared at his uncle as he emptied the cup. After refilling it for him, he braved a question. "What happened to you?" he asked.

Kariya relaxed a little and jabbed a thumb at his face. "Oh, this? This is the price I paid for power."

His uncle was a mystery, one that he couldn't figure out. Why had Kariya left the family? To give up the wealth and prestige of the Matou family, it had to be serious.

"Why did you leave? Dad always said you should have been the heir."

"He did, did he?" Kariya laughed again before sobering. "For your sanity, I hope you never have to find out. It's why your father was driven to drink, it's what left me like this."

"Then why come back at all?"

Kariya was quiet for a little while, then asked, "What do you make of your new sister?"

Shinji searched for the words. "She's quiet. Kind of weird. Sometimes she just stares off into nothing."

Kariya clenched his fingers around the glass. "She wasn't always like that," he murmured, sad and distant. "She was bright and cheerful. I came back to free her. She didn't deserve to be made a Matou, and that Tohsaka bastard didn't deserve her as a daughter. I didn't let Aoi go so her children could wind up in Zouken's hands!"

Something writhed under Kariya's skin. And Shinji didn't know if his uncle was speaking to anyone in particular, much less to him, but he knew the name Tohsaka.

"Uncle Kariya?" Shinji asked, an undercurrent of concern.

With some effort, his uncle calmed down, the churning beneath his skin subsiding as well, though the effort left him looking drained.

"Thank you," Kariya rasped.

"Who is Aoi?"

"Sakura and Rin's mother. I was…I loved her. I wanted her to be happy and safe, and I couldn't do that as a Matou. I had to become a nobody in order to spare her this pain. But that wasn't enough."

The writhing began to start up again, but Kariya brought it under control faster this time, clutching his arm as he willed…whatever it was back under his control. With some effort, he scooted the chair back and slid out of his seat.

"Once again, thank you. For the water. For listening."

Shuffling towards the door, the man hacked and coughed, listing from side to side, stopping at the door, a hand on the handle.

"Shinji?" he asked.

"Yes?"

"I'm going off to fight for her, reunite her with her mother and sister if possible. If I don't make it back, please look after her. Protect her as much as you can. Be the big brother she needs. Please, free her from this family's cursed legacy."

"I promise."

Kariya's shoulders sagged in relief. "Thank you."

His uncle smiled over his shoulder, and turned the door, pulling the hood back over his head. With that, Kariya stepped out of the house, the last time Shinji would ever see him.

I promise.

[F/R]

Despite his uncle's warnings, Shinji's curiosity prompted him to investigate the matter, leading him to the basement one fateful night. In that one moment, he understood everything his uncle had said.

Writhing and skittering worms, so many that it formed a sea in the basement. This was the legacy of the Matou family.

At the center of the churning mass was Sakura, worms crawling over and under her, in and out, enduring all of that without so much as a whimper. Or maybe she had once upon a time, but had given up on the hope of ever escaping, of anyone ever caring.

"Please look after her."

"I promise."

His father would be useless, drunk and insensible as he was. And Shinji had no intention of confronting his grandfather as he was now. Even before, Shinji had avoided him, unable to escape the foreboding he always had whenever the withered elder deigned to appear in the commons of the house. Now knowing his grandfather's true nature, he dreaded it all the more, but it was far more difficult to avoid him.

Kariya had run, and in a way, Shinji could understand his uncle's decisions a little better, but that wasn't an option. He had promised to look after Sakura in his uncle's stead, and running away would spit upon the man's final wishes.

Shinji needed to prepare, to study, and if possible, acquire allies.

[F/R]

Three years later, when Byakuya died, the best Shinji could describe it as was "uneventful". Sakura had never felt very much for the man and Shinji had come to terms with his father's fate long before he had died. A broken body healed readily enough, but a broken mind was much more difficult to repair. Byakuya Matou had been a dead man walking for years.

Before the official funeral, Shinji made a request to his "grandfather". As much as he had come to disdain what his father had become in his final days, Shinji had once held affection towards the man, and salvaging the body for parts seemed a better option than letting Zouken have it completely.

Unsurprisingly, Zouken had laughed. What was surprising was his acquiescence, but even that made sense within the old worm's world view. It wasn't like he was risking his heir, and if it worked, he could derive use from "defective materials". The old worm had made the arrangements before the day was out.

Shinji wasn't going to delude himself in thinking he could outwit the Worm-That-Walked, much less overpower him. The thing was old, but canny and experienced in manipulating everything he came into contact with, and that was before accounting for the power and resources the worm had amassed over the years. Without a doubt, Zouken would try to leverage him, control him, twist him into whatever path the worm pleased.

Outright victory was impossible, retreat or submission were unpalatable, so Shinji consigned himself to the act of apparent appeasement.

So, here he was, strapped to a table in the worm pit, which was mercifully empty, teeth clenched tightly, gritted in pain even with the anesthesia. The procedure was meant to turn him into a magus, however artificially, using his father's atrophied magical circuits. It was beyond risky, not only to himself, but there was a chance that the circuits would degrade or be destroyed in the transfer.

It was like having molten lead poured into his veins. Every nerve screaming in protest as the process dragged on. Every time he felt like giving up – not that he could – he reminded himself of what he promised Uncle Kariya.

I won't die today. I won't die tomorrow.

Live for her. Live for Sakura. For a future.

Sakura had dealt with worse. Uncle Kariya had dealt with worse. Shinji would endure this and more, and if fates were kind, he would drag the Matou family from the depths and see it cleansed and restored.

[F/R]

While recovering in his room, Shinji ruminated recent events and plotted his next course of action.

The circuit transfer process had been finished. Twenty-seven days for twenty-seven circuits. Four of them would prove to be defective after the implantation, but it was far more than he would have dared to hope. His Sorcery Trait finally found its use, unorthodox as it was. The power he had available to him was next to nothing compared to those with more established bloodlines, but it was something he could work with.

Downside, his health had taken a serious hit. A goodly chunk of his hair had turned grey during that month, though that his appearance wasn't a concern in the grand scheme of things. His physical strength, on the other hand, would need some time to recover, and would likely never reach what once was. Some days it was hard to do much more than roll over in bed, let alone anything beyond that.

On the plus side, the Worm-That-Walked wouldn't be able to directly interfere with him too much, and he wouldn't have to suffer Zouken's "training" regimen, as that would likely kill him. Cruel as he was, Zouken was not wasteful, and Shinji would wager that the Worm-That-Walked would want to test him, possibly against another mage, before he tried anything.

Managing to get himself into a sitting position, Shinji accepted the bottle of water from Sakura.

Throughout the process, his sister was a steady companion, visiting his bedside whenever she could. Judging by the look in her eye, she was concerned, and after what had happened with Uncle Kariya, Shinji could hardly blame her.

Shinji mustered a smile.

"I'm sorry for worrying you, Sakura," he said, and patted her head.

"Nii-san…"

"Don't worry. I'll get stronger. Just you wait and see."

He had made a promise. As the future head of the Matou family, his pride as a magus, and for the sake of him and his sister, he had a duty to see it done, however dangerous the road ahead would be.

A/N: For the purposes of this story, Shinji will be significantly less of an a-hole. Still smug, arrogant, and occasionally obsessive, but not antagonistic towards Shirou or Sakura. He still has a beef with Rin, but that manifests in the form of a rivalry and a campaign of petty vengeance.

The concept was partially derived from Shinji's appearance in The Sage's Disciple, and from Born of Prayer's "Kariya Matou's Legacy" challenge.

Questions? Comments? Concerns? Let me know in the reviews or a PM. Constructive criticism is always appreciated.