Jin was crafting a magi-device capable of applying Toughening.
"As long as the magi-reactor's efficiency is high enough, miniaturization should be possible."
"...Jin-nii, wouldn't it be better not to let magi-reactors become commonplace?"
Elsa, who had been watching from the side, chimed in.
"Ah, good point. In that case, let's configure the mass-production model with a mana tank."
"Mm, that's better."
And so, Prototype No. 1 was completed.
In appearance, it resembled an old-fashioned flashlight running on two C batteries with a miniature incandescent bulb. Not an LED.
"To make the area of effect easy to see, I designed it so that Toughening applies to whatever surface the red light illuminates."
"Good idea."
He considered testing it on a nearby piece of stone, but the light spread too wide, revealing a clear drawback: it was ill-suited for small objects.
"Let's make the range adjustable."
With that, he modified the device to allow zooming focus.
"This should do it."
He illuminated a one-meter square block of stone with the red beam.
"When the Toughening process is complete, the target glows red."
"Easy to understand."
"Thicker objects take proportionally longer. The treatment gradually works its way from the surface down into the deeper layers."
The light had to remain focused on the target throughout the process; stopping halfway would leave only the surface or shallow depths treated with Toughening.
"Ah, so that's why it glows red when finished."
"Exactly."
The stone was ten centimeters thick, and the process took about ten seconds to complete.
"I could make it faster, but then it would be harder to control if you only wanted a surface treatment."
"True."
For now, the magi-device that cast Toughening was complete.
"If we build this around mana tank specifications, it can be mass-produced... Wait a minute."
"Jin-nii, what's wrong?"
"No, I was just thinking... if it can apply Toughening, it should also be possible to cancel Toughening."
No such magi-craft existed, but Jin instinctively felt it would be easy to create.
"On top of that, we could probably make a magi-device that conversely weakens molecular bonds... This is bad."
"Eh?"
"I realized how easily it could be abused. Up until now, only those trained in magi-craft could apply Toughening. But if this magi-device spreads, anyone will be able to do it."
"Mm."
"And that means it opens the door for anyone to do the exact opposite just as easily."
Jin explained that with a magi-device developed to this stage, analyzing it would easily allow someone to derive magi-craft with the reverse effect.
"In other words, magic that weakens molecular bonds and crumbles matter to pieces."
"...I think that's definitely possible."
"Indeed, you have arrived at that conclusion once again, My Lord."
At that moment, Laojun's voice rang out.
"Laojun? What do you mean by 'once again'?"
"Four hundred years ago, My Lord, you reached the exact same conclusion and sealed this magi-device away."
"Is that so?"
"Yes. Elsa-sama requested memory adjustment, so it is likely she did not recall anything regarding this until hearing it just now."
"That's true. I remember now... Jin-nii had the same idea and reached the same conclusion at the age of forty-three... Though back then, you hadn't thought of using a red light."
"I see..."
Jin felt a strange sense of conviction; no matter the era, he was still himself.
"Then I suppose the Toughening magi-device is scrapped... Reiko, did you know about this too, by any chance?"
"Yes, Father. I am terribly sorry."
Usually she would offer some kind of comment, yet she had helped in total silence until this very moment. Realizing why, Jin nodded with quiet amusement.
"So the me from four hundred years ago thought along the exact same lines, huh."
Knowing that he had always been fundamentally the same brought a small smile to Jin's face.
"Still, if that's the case..."
Jin tried to imagine what his past self would have done next.
"Laojun, didn't I end up creating some kind of magi-craft like 'Embrittlement'?"
"Yes, My Lord. You did indeed create it."
"I knew it..."
Magi-craft that negated the effects of Toughening and made materials brittle.
Its legitimate use would likely be demolishing buildings or digging up Hardened road surfaces, but...
"It could be used for sieges."
Cast on castle walls or gates, they could easily be breached.
"If used on a golem... the results would be terrifying."
Even metal golems might be shattered by a mere human punch.
"...Which means the me from four hundred years ago came up with countermeasures as well, right?"
"Yes, My Lord."
The "current" Jin reasoned that his past self must have devised a way to ensure it couldn't be used against the forces of Hourai Island, and he was proven right.
"It must be magic cast right after Toughening to Fix the effect, isn't it?"
"Yes, precisely as you say."
"And besides... Strengthen can't be undone by magic that dispels Toughening, can it?"
"It is just as you say."
"I see."
The "current" Jin deliberately avoided looking up the original's achievements.
There had been a time when he wondered if knowing would be better, but he had chosen not to.
"If I already know everything, there's no fun in it," he had said.
Of course, if the "current" Jin were ever about to wander down a path of excessive redundancy, Laojun or Reiko was instructed to step in and point it out.
Jin quite liked this arrangement, so it was likely to continue for some time.
"In the end, it's time for the Smiths to step in."
They fulfilled all the criteria: capable of defending themselves against most threats, and incapable of being complicit in wrongdoing.
By having the Smiths assist, the hotel under construction in Toka Village would surely be built safely and reliably.
"The problem is the cost."
Considering the Klein Kingdom likely had no budget left for such things, Jin pondered for a moment.
"Should I support it covertly...?"
Or perhaps frame it as aid from the Nord Federation. Jin judged that the latter would seem far more plausible.
"We can simply say we're providing a bit of assistance since they're offering the venue."
"They will likely accept that explanation for the time being."
Laojun agreed that this was the better approach.
"All right."
No time like the present. Taking Reiko with him, Jin traveled to the Nord Federation via Warp Gate to pay Sion a visit.
"Welcome, Jin. So, what favor are you asking of me today?"
"Right off the bat, you assume I'm here for a favor."
"Oh? Am I wrong?"
"...Well, no, you're not."
And with that, Jin began explaining the reason for his visit to Sion.