"It really is him."
Black Zetsu was conflicted, feeling both happy and uneasy. Shirou's appearance in this era was an accident, and he had ultimately betrayed it in the ancient era, but Black Zetsu still missed the time they had collaborated.
It was a strange feeling, like meeting someone stunningly brilliant when you are at the lowest point in your life.
Whenever it thought of Emiya Shirou, it remembered their first meeting.
At that time, Shirou was trying to assassinate Indra. At that time, Indra still only had a three-tomoe Sharingan. At that time, Black Zetsu thought some minor character was seeking death. But that battle had nearly scared it to death.
This minor character had somehow obtained the remnants of the God Tree, mastered Wood Style and natural energy, and possessed formidable taijutsu and kenjutsu.
With a small slug by his side, he had beaten Indra so badly he couldn't even lift his head.
Because it was secretly following Indra, its attempt to help caught Shirou's attention, which made him give up on the assassination.
Later, it failed to escape, and so it got to know Emiya Shirou. It learned he wanted to destroy the Ninshu founded by the Sage of Six Paths.
When the Sage grew old, the philosophy of Ninshu had spread. Some disciples used chakra to benefit their regions, while others grew greedy and used it to seize power.
Shirou was one of the victims. A Ninshu disciple, to steal a few bottles of good wine, had used chakra to slaughter an entire village. Shirou was spared only because he had been away. To take revenge, he prayed to the remnants of the God Tree and obtained the power of Wood Style.
The power was strong, but to challenge the Sage of Six Paths with it was a fool's dream. Black Zetsu paid little attention to this pawn, only knowing that he was saving people everywhere while spreading the belief of medical ninjas.
At that time, Black Zetsu was at the height of its success. Its first plan had succeeded: it had instigated Indra to challenge Asura, hoping he would take Asura's power and awaken the Rinnegan. But Indra was soundly defeated. The plan failed. Dejected, Black Zetsu went into hiding.
While it was dejected, Shirou was still saving people, proclaiming that chakra was a power to relieve suffering, not inflict it. In that era, Shirou's prestige surpassed even the Sage of Six Paths.
The people had always believed in the God Tree, but the Sage had destroyed their faith. In comparison, Shirou, who was constantly saving people and also believed in the God Tree, was worthy of respect. Even the Sage and his son Asura admired his beliefs.
Shirou created the philosophy of the medical ninja: enduring the world's suffering while striving to save those who could not escape it. This benevolent belief naturally earned the admiration of the morally noble Sage.
Even Black Zetsu found that Shirou was truly a kind and good person.
Of course, adhering to the principle that even bad people like good people, Black Zetsu very much liked a good person who could be used. In those lonely years after its failed plan, it felt it had found a kindred spirit, tirelessly speaking ill of the Sage in Shirou's ear.
They criticized the Ninshu disciples who abused chakra. Among these discussions, many secrets were told. For example, that the Sage of Six Paths was the source of chakra. This wasn't much of a secret, but it piqued Shirou's curiosity.
After the Sage died, Shirou wanted to dissect his body to find the source of chakra, hoping to find a way to retrieve it from those who brought suffering to the world. He wasn't after power. As a medical ninja, studying human organs was necessary.
He was the first person to dissect a corpse to improve his skills, and he had told Black Zetsu more than once that he wanted his own body to be made into a specimen for future generations.
Black Zetsu informed him of the whereabouts of the Sage's body. Due to the Sage's immense power, it didn't dare approach, but Shirou had no such scruples.
He deftly stole the body and took it to a place rich in natural energy called Shikkotsu Forest to begin a long period of seclusion. Black Zetsu did not dare to approach the deadly, acid-filled forest.
When Shirou appeared again, accompanied by a huge slug, he possessed part of the Sage's power, having successfully transplanted the Rinnegan. Black Zetsu had never thought humans could achieve such results through research.
Before it had a chance to guide Shirou, he had already taken action, defeating the entire Ninshu in one fell swoop. He killed countless disciples, turning their village to ruin. He was only one step away from destroying everything the Sage had founded.
That was the time Black Zetsu's plan came closest to success. It was its most satisfying revenge on the Sage, a physical and spiritual execution of Ninshu.
But in the end, Shirou lost to Asura.
He had met Asura before, while dealing with the God Tree's remnants. He had seen Asura helping villagers dig wells and believed he was a good person. Under Asura's earnest persuasion, Shirou chose to believe his promise, that Asura could lead everyone to save the suffering world.
This man decided to entrust his dream to Asura. To atone for his sins, he even used the Rinne Rebirth Technique to revive those who had died in battle. He did not die on the spot, but returned to Shikkotsu Forest to prepare for his passing.
When he appeared again, all that was left was a crystal coffin with half a corpse, which the slug entrusted to Asura.
What a naive person, to actually believe Asura's promise.
However, Shirou's success made Black Zetsu discover another possibility. For a thousand years, it worked to provoke battles between the descendants of Indra and Asura, while also spreading the greatness of medical ninjas, hoping someone could master human experimentation like Shirou.
This was a source of frequent confusion for Black Zetsu: it was a murderous merchant of war, yet also a holy messenger of healing.
The fact that Shirou's story was passed down while the Sage's became a legend was in no small part due to its efforts.
Unfortunately, neither its own methods nor Shirou's had ever brought it close to success again. It was for this reason that Black Zetsu missed Emiya Shirou even more.
He was the one time, among its countless plans, that had come infinitely close to success. What a regret. If only he hadn't believed Asura.
"Heh heh heh..." A mocking smile appeared on Black Zetsu's face. It was imagining meeting Shirou again, imagining the regret and shame for his past betrayal that would be on his face.
Seeing the endlessly fighting ninjas, do you now regret believing Asura's foolish words?