The rule for becoming a special material is that it inherently possesses a special ability or is the subject of a rumor or meme. If it is a person, then their personality and experiences will also play a part.
For example, Uncle Teuchi.
That "Behind-the-Scenes Boss" item was created because back when the anime had not yet finished, many people guessed that he was the ultimate boss, and some also guessed that he was the Sage of Six Paths, an important character who would give Uzumaki Naruto a power-up later on.
Later, when the anime finished, this guess became a widely spread meme, with many netizens photoshopping pictures of Six Paths Ichiraku and Otsutsuki Ichiraku.
I heard that the anime company even created an original anime episode after learning about the meme, though I do not know if that is true or false.
These memes caused the bodily tissues of Uncle Teuchi, who was an ordinary person, to possess the trait of forging special items.
Uchiha Itachi's abilities and memes are quite numerous.
Things like instant-killing all creations, one against seven, foolish little brother, the Four Great Laughers, and many more.
The blood he left here is also not a small amount. Jokichi had long expected to be able to forge quite a few items, it was just...
The Broken Sage Crystal and the Mangekyo aside, what the hell are this Sword of Filial Piety, Brotherly Bonds, Heart of Peace, and Never-Ending?
Where is my Totsuka Blade that can seal all things?
Where is my invincible Yata Mirror?
Where is my Yasaka Magatama that is comparable to a Tailed Beast Ball?
Could it be that these three items are visual jutsu inherent to Uchiha Itachi's Mangekyo Sharingan?
But Orochimaru clearly stated that the Totsuka Blade is a type of the Sword of Kusanagi, and he had been searching for it. This at least proves that the Totsuka Blade is a treasured tool, not a visual jutsu.
Is it that Uchiha Itachi has not obtained these things yet, or is this blood not enough to forge everything?
Or is it that external items cannot be forged through the owner's bodily tissues?
The things he wanted did not appear; instead, a bunch of items with messy names appeared. Jokichi only wanted to swear.
However, thinking of the neighbor aunt who had just been woken up and went back to sleep next door, he forcefully held it in.
"Forge."
He silently muttered a sentence in his heart, and then the blood disappeared, and six items appeared in his hands.
A thumb-sized, blood-red crystal-like Broken Sage Crystal.
A pair of Mangekyo that looked no different from ordinary contact lenses.
A Heart of Peace shaped like a heart, emitting a soft light, alternating between transparent and solid.
A Sword of Filial Piety, a tachi about 1.3 meters long including the hilt, with a blood-red blade exuding coldness.
A pair of ordinary-looking silver ear studs, Brotherly Bonds.
A pitch-black, constantly swirling mass of gas that seemed to have no end, Never-Ending.
Jokichi observed the appearances of the few items for a while but could not figure out anything special about them, so he immersed his mind in the information provided by the system.
[Broken Sage Crystal (Upgradeable): After the crystal is swallowed, it will change the user's brain, transforming their eyes into degraded Sage Eyes.]
[Mangekyo (Upgradeable): These are contact lenses of degraded Sage Eyes; the wearer can use the abilities of the Sharingan.]
[Heart of Peace: A miracle triggered by an intense desire for peace, capable of resurrecting the dead. (Note: Can only be absorbed by a deceased person who loves peace and maintains an intact body. Limited to one person.)]
[Sword of Filial Piety: After the wielder kills their parents, they will gain immense power, but at the same time, they will bear an unforgivable sin forever. (Note: Only effective for someone with a harmonious family and a deep father-son relationship.)]
[Brotherly Bonds: Both parties wearing the ear studs can mutually sense each other's feelings and thoughts.]
[Never-Ending: After absorbing this, one will bear an incomparably heavy weight. The weight cannot be canceled and will increase over time until death.]
[Ding! Detected that the host possesses two Broken Sage Crystals, they can be fused and upgraded. Do you wish to upgrade?]
[Ding! Detected that the host possesses two pairs of Mangekyo, they can be fused and upgraded. Do you wish to upgrade?]
Jokichi: "..."
Jokichi fell silent.
If it were a normal day, he would definitely complain about a few useless items, but right now, his heart was heavy, and he could not speak.
Special materials possess special abilities because of their inherent traits.
If the special material is a person, it will be formed by that person's experiences, personality, and rumored memes, and the forged items are also generated based on these conditions.
These six items fully reflected Uchiha Itachi's current physical and psychological state, as well as the situation he was facing.
Extraordinary abilities, a harmonious family, a desire for peace, bearing a heavy burden, and... having made the resolve to kill his parents amidst being unable to make a decision.
"Phew!"
Jokichi took a deep breath, then slowly exhaled, his gaze becoming increasingly determined.
"This is the embodiment of being weak and unable to determine one's own destiny. Just like back then when I could only lie in swaddling clothes watching the collapsing house bury me, watching the Nine-Tails step over my head from the cracks. This feeling of powerlessness..."
"I absolutely do not want to experience it again!" Jokichi clenched his fists tightly.
"Upgrade Mangekyo."
The Mangekyo in his hand and the Mangekyo worn in his eyes automatically flew out, then attracted each other and pressed closely together.
Following that, a soft light bloomed, making it impossible to see the situation inside clearly.
After about five seconds, the light disappeared.
A new pair of contact lenses was revealed.
Jokichi reached out to catch them, and the information of the new Mangekyo appeared on his retina.
[Eternal Mangekyo: These are contact lenses of degraded Sage Eyes; the wearer possesses the abilities of the Sharingan.]
Except for the name, the rest of the introduction did not change.
Jokichi put on the glasses and immediately knew the specific changes.
It was roughly the Eternal Mangekyo Sharingan after Uchiha Shisui's and Uchiha Itachi's Sharingan fused with each other.
The glasses simultaneously possessed the visual jutsu of Susanoo, Tsukuyomi, Amaterasu, and Kotoamatsukami.
Moreover, because of the fusion of the two, the visual prowess became even stronger.
All abilities underwent changes and became stronger.
For example, Susanoo.
Originally, the Susanoo used by the Mangekyo forged from Uchiha Shisui's blood was green. After upgrading to the Eternal Mangekyo, the Susanoo used became a mix of red and green, simultaneously possessing the characteristics and abilities of two Susanoo.
Furthermore, before, he could only use an ordinary Susanoo, at most covering it with a layer of armor. But now, he could use the Perfect Susanoo, the kind that grows a Tengu nose and wings.
However, there was no Totsuka Blade, nor were there the Yata Mirror and Yasaka Magatama.
"It seems these three are not visual jutsu, but treasured tools that Uchiha Itachi will find in the future." Jokichi guessed.
But even so, once the Perfect Susanoo comes out, no one in Konoha of this era could block it except for Might Guy opening the Eight Gates.
Of course, using Susanoo brings immense pain and burden to the body. With Jokichi's current physical fitness, even if it were a skeletal Susanoo, it would instantly crush his body.
Besides Susanoo, Amaterasu's activation time became faster, and Tsukuyomi's maximum casting distance changed from five meters to seven meters.
And Kotoamatsukami's cooldown time changed from over ten years to five years.
After understanding the effects of the Eternal Mangekyo, Jokichi also roughly knew the effect of the fusion upgrade of the two Broken Sage Crystals.
It was nothing more than changing from possessing the Uchiha bloodline and awakening Uchiha Shisui's Mangekyo Sharingan to a crystal that possessed the Uchiha bloodline and awakened the fusion of Uchiha Shisui's and Uchiha Itachi's Mangekyo, upgrading to the Eternal Mangekyo.
Jokichi remembered that in the anime, it required the fusion of brothers' Mangekyo to upgrade to the Eternal Mangekyo, but the God-Level Forging System changed the condition of needing a brother's Mangekyo to any Mangekyo would do.
However, the anime did not explicitly state that it had to be a brother's Mangekyo.
It was just that Uchiha Sasuke's and Uchiha Madara's Eternal Mangekyo were both upgraded by transplanting their brothers' eyes, so whether absorbing other people's Mangekyo visual prowess could upgrade it was an uncertain factor.
Perhaps it might have been possible to upgrade it this way originally.