FATE/Probability

Shirou prepared snacks and tea for the two Fujimura men but they instantly left after kiritsugu and Taiga went to the living room with the books.

Helpless Shirou could only go and put the plate on the table near the studying pair and was instructed by Kirtsugu to not disturb them for 2 hours.

Shirou went to do his things after this still thinking about his magic circuits awakening.

Finishing his work he made dinner for Kirtsugu and went to bed.

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"Shirou wake up! it's 11:30 pm." 

His father`s call woke up Shirou, who jumped out reflexively thinking it was time for a morning workout until he remembered about the magic circuits. 

Freshening up Shirou with only shorts was pulled into the garden. There was an elaborate magic circle much more complex than the one used for summoning servants drawn on the ground.

" Shirou sit in the middle of the ritual. I will activate it sharply at midnight." said kiritsugu 

"And don't thrash around during the process, and be prepared for pain similar to filling your nerves with lava." cautioned kiritsugu 

"I am ready," replied Shirou with determination evident in his voice. 

"I see, I sincerely hoped for you to not walk down this path Shirou but as my origin says I can never mend my mistakes." said kiritsugu with a sigh and walked into the ritual field and powered it up.

They waited till midnight and the moon peaked out from behind the clouds. As rays of moonlight landed on the magic circle kiritsugu poured his prana into Shirou's body.

Felling the existence of channels attracting the mana kiritsugu flooded those channels forcefully breaking them open.

Shirou felt a gentle warmth when his father poured mana into him until it started to sting at some places which he couldn't describe making him release a grunt.

Then he started feeling as if someone was slowly pooling boiling water on his back when mana was directed into his circuits.

"ahh!"

 Shirou released a bloodcurdling scream when his circuits were flooded with mana and his arms as well as the left side of his face started flickering with greenish circuitry.

Shirou held on even when it felt like there was molten metal being poured into his veins. What seemed like days for him finally ended with the scalding feeling being replaced by a warm flow.

He felt he heard a hammer being struck on a red-hot blade. He pictured a black iron hammer with a brown wooden handle hitting a red hot piece of metal and small fragments splashing about before he started feeling sleepy.

The older emiya detached his hands from the boy's back and picked him up. Then he said 

"It's ok now Shirou, you can sleep."

Smiling Shirou replied 

"I can now be a magus like you Dad." and closed his eyes.

Looking at his son who fell asleep from exhaustion kiritsugu looked at the moon and thought 

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…

Shirou woke up late the next day. Looking at the warm winter sun and slight snow he thought of the previous night and the ritual. 

Fresh from the bath and wearing fresh clothes, Shirou walked into the living room and saw his father writing something in a notebook. Looking at Shirou kiritsugu asked 

"You are awake. Now tell me do you feel discomfort anywhere in your body."

"I feel very refreshed and full of energy," replied Shirou with a happy smile.

"Good, then you can start your morning exercise now." said kiritsugu while pointing at the lawn visible through the open window.

Shirou`s smile just froze on his face and he went to exercise.

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…

After having lunch the father and son duo moved into the room with the bounded field. Looking at Shirou sitting with an eager expression kiritsugu said 

"As I have explained to you numerous times magic circuits are like an extra pair of limbs for magus, hence we have to set up a mental trigger to open them."

"It is a form of self-hypnosis to get into the right state of mind to open the circuits."

"What is this right state of mind and how do I reach it?" asked Shirou.

"That is a unique imagery that you have to put in your mind when opening the circuits. It acts like a switch for the circuits." said kiritsugu and continued while pointing at Shirou.

"You just have to remember the image that came to your mind last night when I opened your circuits, and you must remember that the next few times you open the circuits it will hurt less progressively."

"Now mediate here and try to open your circuits."

Shirou listening to the instruction given to him closed his eyes and started searching for that feeling from last night.

Time continued to pass and Shirou became lost in different feelings until his head was devoid of thoughts. 

the image of an anvil carrying a flat block of iron came to his mind naturally, when a common-looking hammer struck the block with a resounding clang he felt a burning pain.

Shirou felt that his blood was being replaced by molten metal and was flowing. His body`s nerves went haywire and he felt as if he was slowly being burnt and thought 

With this thought the glow of circuitry on his body dimmed and his clothes drenched in sweat stuck to his body. Pulling his attention away from himself kiritsugu`s voice sounded 

"If this much pain is enough for you to lose focus then how will you survive in this world? If you can't bring your circuits opening under your control within 3 days then it would be better for me to send you to the atlas."

Hearing his father`s criticism the images of the 4th holy grail war flashed in his mind. He remembered the fight between the lancer and the sabre.

 He remembered his dad targeting others with sniper. The people getting killed by the mad caster and the words of the master of lancer to the rider`s master.

Remembering this strengthened his resolve and he answered.

"Yes, I will do it within 2 days"

Leaving Shirou to practice alone kiritsugu went to the living room.

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Fiddling with the notebook he wrote as taiga`s homework kiritsugu though 

< Shirou I don't know what should i say about your luck, having 27 magic circuits of average quality as a non-magi descendant is rare and your circuit numbers are above the 20 average but your lack of quality in circuits will make your path much harder than necessary.>

< I have to arrange for you to go to Clock Tower to learn magecraft after I die. Till then I will train you enough to be capable of self-defence.>

Pulling him out of his thoughts the doorbell rang.

Standing up he went to the door and found Taiga Fujimura his student who resembled Shirley, making him feel as if fate was playing a cruel prank on him by severing and binding his life.

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On the 2nd day after opening his circuits, Shirou was finally able to control his circuits after listening to the advice of kiritsugu.

He thought back to what Kiritsugu said

"Associating the triggering of your circuits with any sensation is the path to ruin, if you do that you will only be able to use your circuits when feeling that sensation as well as you can`t stop the opening of your circuits when you receive that sensation."

"Depend on your mental trigger only and don't associate the opening of circuits with pain or heat." 

Following this advice after opening and closing his circuits for a day, Shirou was finally able to open and close them under control when he concentrated.

Upon telling about his achievement to kintsugi he plainly said 

"Now that you have achieved the basic requirement I will train you to keep your circuits open even when you are distracted and then we will learn mana sensing."

Next, kiritsugu proceeded to instruct Shirou to open and close his circuits randomly while doing work. He was even trained to keep his circuits open while exhausted from exercise and even during it.

By the end of the week, Shirou was able to open and close his circuits perfectly. It has become a muscle memory for him.

He no longer opened them by mistake or suddenly closed them when distracted. 

During this week he also met Taiga Fujimura who came to be tutored by kiritsugu 6 days a week. She insisted he called her sister Taiga not unlike how Mr. Raiga Fujimura insisted on being called Grandpa.