Sengo was seven when he was brave enough to check his Magic Circuit and open it himself.
It all started when he accidentally opened one. He had been practicing a sword then, it was nothing special, just some basic sword move. But he keep repeating the same move, [Vertical Slice], with a one edged wooden sword that he made himself. It resembled a Katana, of course, since he was trying to revive Senji's swordmanship through himself.
Everyone had been confused, but his father seemed to understand what it was and had helped him in crafting the wooden sword.
Anyway, it had been when he had swung the sword for exactly 87 times when it happen.
He had been tired, his arms were aching, his body wanted to stop, but Sengo keep on going, but just right when he was about to reach his limits, something in him awaken.
It felt like water 'crawled' underneath his skin, and when he swung down the Wooden Katana, he felt himself powering it up with that 'water'.
The result was a powerful swing that he shouldn't been able to do at his current level. One that somehow cut a line in the ground he aimed his Sword at. A cut that could've hurt someone, procuring a cut on their body, albeit a swallow one.
He look at his Wooden Katana, and then draw the feeling once again. He... 'Reinforced' the Wooden Katana, made it sharper, filling it with power, more and more... But that seems to be a bad move considering the Wooden Katana exploded.
If it wasn't for his instincts that he carried from his past, he could've been hurt badly. He was still hurt, especially his arms since the wood splinters were embedded there. And it also alarmed the Guards, who had been practicing and sparring while he was training his basic.
They rushed towards him, worried etched to their faces, but Sengo didn't pay them any heed. No, he had just done something unthinkable, something that changes his life from this point on wards.
The discovery that he have Magic Circuits.
--
Sengo started another training after the discovery.
After knowing that he had Magic Circuits, he needs to check just how many and what their qualities are, and then he needs to figured out what his Affinity and Origin was. This would determine what he would do next before starting his Magecraft training.
Of course, he needs to be discreet, considering his mother's worry from back then when he had been hurt by Reinforcement's side effect, had she known the real reason she would worry so much that he wouldn't be able to leave her sight. He wasn't even allowed to practice swordsmanship for a few weeks because of that accident! Fortunately, he just need to fall into a meditative state to look inside himself.
Truthfully, he hadn't tried to look inside in the past, he just meditate to calm his state of mind from all his training. Thinking about that make it seem like he was obsessed with getting stronger...
He had been obsessed with getting stronger.
Sighing internally, Sengo decided to ignore that realization and went to work.
Sengo emptied his mind from worries and all thoughts, he focused. Once his mind was empty, Sengo pulled himself inside his mind.
When he 'opened' his eyes, he didn't see anything. It not that there were nothing.. No, it was more like he was right at the door to his destination but he couldn't reach that door.
He decided to ignore that for now, things will add up later, he was sure.
For now, Sengo focuses on something else, his body. He spread his awareness to every part of it and check his body, physical and non-physical. What he found was quite astonishing.
If he were to put it in the 'Moon-lit' World's power system of Magic Circuits, he could count up to 54 Magic Circuits inside his body. Also, he didn't know if it was because of the different setting or world, but as he finally able to take notice, the mana in the air seems to be absorbed by his Magic Circuits efficiently and faster, the mana absorbed and turned into Od was by a large amount that if he were to count it was around a hundred per unit.
Sengo considered himself a lucky bastard.
If we consider the luck that Shirou Emiya had and Senji Muramasa's own luck, you'd think they were the most unlucky men. Maybe it had been because of their dreams, but Sengo was sure, after watching their lives until he could recite a line in one of their conversation, they were just that unlucky. And then, there was him. He wasn't unlucky, per se. He was closer to being average when it come to luck. He was sure that it involves multiplication and division thing, about a minus times minus equal positive? Yeah, that.
He was sure it was that. Nothing else make sense, really. Not to him at least.
Well, he could think of another option... That voice. He was sure it was a God or some ROB. That being wanted him strong just so he could help it's 'child' in their time of need. Whoever that is..
Sengo was done checking. Now, he needs rest. The injury and mental shock was trying him and he decided that he want to rest. Something about that sounds good... Besides, he doesn't need to hurry much. It's not like the whole world is ending in a few years when he could be 20 or so years old.
...
On second thought, that wasn't an impossibility.
But he was tired, so he put himself to bed and went to sleep. The End of the World thoughts can wait for a few more hours.
--
Sengo find himself in the forge a few days later. Today, his father was going to teach him the basic of forging.
This event came up because Sengo had been holed up in his bedroom since Sarah doesn't seem to allow him to do anything but stay in bed. The kid practically look dead out of boredom, so Ryugo decided to pull him out and bring him to the forge to learn.
The reincarnated was honestly confused and more intrigued, especially when his father handed a hammer, albeit a smaller version of his father's hammer.
Though Sengo figured sooner than later that his father was going behind Sengo's mother back. It was risky, but Sengo didn't waste the opportunity to forge and eagerly learned the basic of forging from his father.
First, his father taught him about Wrought Iron, Carbon Steel, and many raw material for forging and also all the tools needed for it. Sengo learned a lot from here, even though he already knew what those are, learning about it in more details and with his own understanding was far better than learning from the memories of Senji Muramasa or Shirou Emiya.
After that, his father taught him how to properly heat up each raw material. But for now, Ryugo uses Wrought Iron, as it was softer and brittle enough for practice. His father also verbally added a way to weld steel and Iron, seeing as the mix of Wrought Iron and Carbon Steel was a perfect material for a good knife and sword. Which Sengo keep in mind.
Ryugo then proceeded to the next step, which was determining the weight of the material before they hammer it into shape. All of which Sengo was absorbing like fish to water. This was considerably easier to learn than anything else he was learning for the past year.
And then, his father finally taught him how to hammer properly. Ryugo taught him how to distribute his strength and how to weld the steel into the shape he wanted it to be. All those sculpting bore fruit at this moment, he was considerably elated as he use the hammer to shape his first creation. Insignificant as it was, it was also the first step towards his goal.
And while at it, his father also taught him tampering, when to know to cool the Iron down and when to heat it up again.
It took Sengo three hours to make a brittle knife out of Wrought Iron. Truly, it was insignificant and not worthy to be used to cut something or could be used at all, but it was his first try and he was happy it even look like a knife.
It was exactly when he was inspecting his first forged Iron Knife that his mother arrived.
And that was how he and his father got lectured by Sarah for taking Sengo out of bed and make him do labour like forging considering he was still six, albeit almost seven, but semantics.
(That Iron Knife was put on proudly on display on the forge wall by Ryugo and was never taken down, even after Sengo took over the forge)