Sometimes, words require a certain finesse.
Emiya Shirou couldn't just tell Senju Tobirama that he could cure his brother if only he were truly resurrected. Tobirama was suspicious by nature; such a direct approach would immediately make him suspect the entire situation was a conspiracy.
Which, of course, it was.
But precisely because it was a conspiracy, it needed to be woven with care. A clumsy plot is easily unraveled.
However, Senju Tobirama proved to be a bit insensible. He completely glossed over the hint, his mind latching onto the one thing that mattered to him: the "Yang Power" that could solve his brother's lifespan issue.
"Lord Emiya, what is this Yang Power?" Tobirama's brow furrowed in concentration as he began to analyze it himself. "This power seems related to Yang-natured chakra. I recall that Yang chakra can indeed create many miraculous secret techniques. The use of medical ninjutsu itself is related to it." He thought of the Akimichi clan, who used a secret jutsu derived from Yang chakra to vastly increase their size and strength.
"That's right," Shirou affirmed, forced to elaborate. He tried his best to entice Tobirama with the possibilities. "Yang Power is the ultimate form of Yang-natured chakra. It can create life from nothing, regenerate a patient's severed limbs, and allow damaged organs to recover in an instant."
As he spoke, he cast a regretful glance at the still figure on the bed. "Naturally, it can also restore the life force of those whose own has been exhausted, allowing them to continue living."
"Then..." Tobirama leaned in, asking the crucial question. "How can a ninja elevate their Yang-natured chakra to its ultimate form and possess Yang Power?"
"That's not easy." A glint flashed in Shirou's eyes before he sighed softly. "The simplest method is for a person who already possesses Yang Power to share it with another. Your brother," he added, "is the most suitable person for me to share my Yang Power with."
A touch of emerald green chakra appeared on Shirou's palm before dissipating. He said with deep regret, "Unfortunately, the Yang Power in my body vanished during this resurrection. I don't know what happened."
Tobirama took a deep, steadying breath. "My apologies. This is my mistake." He took the initiative to apologize, his voice low. "There must have been an error in my Impure World Reincarnation. This was my first time using the jutsu."
"It's fine," Shirou said, comforting him while simultaneously tightening the trap. "Perhaps it's because this is not my original body, or maybe the power disappeared in the Pure Land. It's impossible to be certain."
Tobirama frowned and nodded thoughtfully.
Is this guy for real? Emiya Shirou screamed internally. You really think the problem might be me? To ensure this jutsu's success, my soul practically camped out by the gates of the Pure Land, ready to jump the moment your summons came through! If I hadn't cooperated, it would be a miracle if your crude technique worked at all!
This was becoming a headache. Since things had come to this, Shirou could only adapt and continue. He had to find another excuse to remain in the living world. Even if he had to operate within a reanimated body for a while, other opportunities would arise. It wasn't as if he hadn't left a few countermeasures in place before his death.
"Forget it," Shirou sighed, looking at Hashirama. "For now, I'll try other methods. First, I'll restrict the chakra in his body to prevent his self-healing ability from dividing his cells too quickly."
"That's not easy," Tobirama said, agreeing with the logic but skeptical of the means. "My brother's chakra is immense. Restricting it is difficult."
"It is indeed very strong," Shirou praised, raising a finger as a wisp of pitch-black energy coiled at its tip. "Not much inferior to your clan's ancestor, Asura."
He looked at the energy. "This is a cursed seal, condensed from natural energy. It can prohibit a person from using their chakra. If they try, it will restrict their movements."
Shirou's expression turned cautious, as if unsure of his own method. "I'm not certain it will be effective on him. His body shows traces of having practiced Sage Mode, so he might adapt to the cursed seal's energy too easily, rendering it useless."
"Natural energy... can you let me feel it?" Tobirama suggested. "I have some understanding of certain jutsu. Perhaps I can see if it can be improved and offer some help."
This couldn't be more perfect. The ancient sage's hesitation was exactly what was needed. Tobirama himself was conflicted; he trusted Shirou, yet he was embarrassed to doubt the cursed seal. But when it came to his brother, he couldn't afford to trust anyone completely. Only by handling it himself could he be at ease.
"I'm just a little worried," Shirou said, a trace of unease on his face.
"Is there a problem?" Tobirama's eyebrows rose.
"This is not a commonly used medical jutsu," Shirou said, voicing a "sincere" concern. "Although the cursed seal will temporarily seal a person's chakra, once they completely master it, it will allow them to gain even more powerful chakra by fusing with the seal. I'm worried... that spreading the power of the cursed seal to the world will lead to it being used for fighting, just like chakra."
He sighed, lamenting the past. "Back when the Sage of Six Paths and Ninshu spread chakra, many were worried it would be used for fighting. Asura and I believed people would use it to help with the world's suffering, just as we did. Looking at it now, our thinking was indeed wrong. In the end, chakra was just used by ninjas to fight each other."
"Lord Emiya," Tobirama said, his voice firm as he comforted the sage, "this is not your fault. In this world, what is wrong is never the power, but the greedy hearts of some people that refuse to rest."
He then raised his palm, his expression cold and resolute. "Please rest assured. I will not leak the cursed seal. Furthermore, if I ever use the power of the cursed seal for fighting, may my soul find no peace in the Pure Land after death!"
The corner of Emiya Shirou's eye couldn't help but twitch.
Wait a minute. Does this guy have any self-awareness at all? The inventor of the Impure World Reincarnation, a jutsu that exists solely to disturb the peace of other people's souls, and you still think your own soul has a shot at resting peacefully? How could you have such an extravagant idea?
However, since Tobirama had made such a solemn vow, Shirou could only go with the flow. He would apply the cursed seal. If Tobirama ever discovered a problem and tried to release the reanimation, the seal would serve as a nice little countermeasure.