(Edited)
The chakra reserves Hashirama referred to were about the amount of chakra. Senjutsu chakra, or sage chakra, is formed when natural energy and one's own chakra are maintained in a specific ratio. If this stable ratio is lost, the Senjutsu chakra dissipates. Worse still, if the natural energy exceeds the user's chakra, the user can be overwhelmed and swallowed by the energy.
Masaki, however, could form perfect Senjutsu chakra due to an inexplicable spiritual energy within him. He didn't need special maintenance to balance the ratio, allowing him to attain perfect Senjutsu chakra effortlessly. In the past, he couldn't control natural energy, and as a result, after all the Senjutsu chakra condensed, natural energy continued pouring in, causing him to become severely ill. But after learning to control the energy, he found that due to his limited chakra, he couldn't condense more Senjutsu chakra or access higher levels of power.
The Fumori clan's bodies weren't particularly strong, and their chakra reserves were meager. Masaki, only 16 years old, would naturally gain more chakra as he grew, but it would take at least another decade to meet his needs. Masaki couldn't wait that long.
Anxiously, Masaki approached the Hokage, asking how he could quickly increase his chakra reserves. At first, Hashirama was confused by Masaki's urgency for power, but after some thought, he understood. Among his generation, Masaki wasn't outstanding, except for his sage skills. For a young ninja, that kind of pressure was significant.
After some guidance, Hashirama grew pleased with Masaki's dedication and serious approach to learning. Masaki's aptitude for sage arts stirred a sense of inherited responsibility in Hashirama. After pondering for a while, he decided to help solve Masaki's problem. It wasn't impossible, after all.
"Well, since you're so eager, I'll give you a suggestion," Hashirama smiled, then said, "If you want to increase your chakra reserves, it will mostly depend on your growth. Every clan has its own methods, including secret medicines."
Masaki nodded. His clan had some secret medicines to aid in training, but he'd taken them since childhood, and they no longer had any effect. The secret recipes from other clans likely worked similarly, except for stronger, harsher medicines, but those could damage the body. For example, the three-color secret medicine of the Senju clan was notorious for its intensity.
Could the Senju clan's secret medicine be different? Masaki looked at Hashirama hopefully, but Hashirama said, "However, for someone your age, the effects of secret medicine, even for our Senju clan, are minimal. But…"
Hashirama's lips curled into a slight smile. "While ninja secret medicine has its limits, there are things in this world beyond a ninja's imagination."
Masaki's disappointment lifted, and he asked excitedly, "What things?"
"Wait a moment," Hashirama said calmly. He bit his finger, drawing blood, then quickly formed hand seals and pressed his hand to the ground. Instantly, a large array of runes spread out, forming a summoning circle. With a puff of smoke, a white and blue creature appeared before them.
"This is..." Masaki quickly recognized the creature. It was a slug, the same one Tsunade had used as a summoning beast.
Slugs were creatures from the Shikkotsu Forest, a place renowned for sage arts. Unlike the other sage locations, Shikkotsu Forest had only one creature: the Slug Sage. All the slug summons were parts of this creature, mere clones.
Although the slug's appearance surprised Masaki, it wasn't entirely unexpected. Tsunade's connection with slugs and her family's ties to the Shikkotsu Forest explained a lot. However, Masaki wondered how this summoning related to Hashirama's mysterious words.
Masaki recalled the toad oil from Mount Myoboku, which helped absorb natural energy, but he doubted it could increase chakra. Of the three great sage locations, Shikkotsu Forest was the most enigmatic. In the stories Masaki knew, the White Snake Sage from Ryuchi Cave had appeared, but the original Slug Sage never did.
Hashirama interrupted Masaki's thoughts. Smiling at the slug, he said, "It's been a while, hasn't it?"
The slug's voice, soothing and gentle, echoed, "Indeed, Hashirama-sama. I didn't expect you to summon me."
Hashirama chuckled. "I need your help with something."
The slug looked curious. "How can I assist you, Hashirama-sama?"
"I hope to borrow something from the Shikkotsu Forest. This boy here needs it," Hashirama said, gesturing towards Masaki.
The slug glanced at Masaki and bowed slightly. "Greetings."
"Hello," Masaki replied, impressed by the slug's calm demeanor. Unlike the grumpy or prideful creatures from Mount Myoboku or the vicious serpents of Ryuchi Cave, the slugs seemed serene.
"Since this is your request, Hashirama-sama, I shall help," the slug said kindly. "Allow the boy to sign a contract with me, and then I'll take him to the Shikkotsu Forest."
Hashirama nodded, and with another puff of smoke, a large scroll appeared. Hashirama unrolled it, revealing many names, with his own name at the end.
"Masaki, write your name in blood and leave your fingerprints. Then I can take you to the Shikkotsu Forest," Hashirama instructed.
Masaki hesitated for a moment, surprised at how quickly this was happening. Signing a contract with a summoning beast from the Sage lands felt surreal. Nonetheless, he bit his finger, wrote his name in blood, and left his fingerprints on the scroll.
"Good," Hashirama said with satisfaction as the slug vanished, promising to return soon.
"Hashirama-sama, why are we going to the Shikkotsu Forest?" Masaki asked.
"Because what you need is there. We could bring it out, but its effectiveness would diminish," Hashirama explained. "This substance is part of the Senju clan's secret medicine. You know its potency."
Masaki was astonished. If the secret medicine of the Senju clan was as effective as it was, whatever they used from the Shikkotsu Forest had to be incredibly potent. His anticipation grew.
Suddenly, Masaki felt his body lighten. A sensation of weightlessness washed over him, and everything around him turned dark. Before he could react, he felt solid ground beneath his feet. He quickly looked around, trying to understand where he was.