Based on the information in his memory, a "contract" should be an exchange agreement guaranteed by a certain world-binding force. The specific details would require further research to confirm, but unfortunately, Hiruko did not have any powerful summoning scrolls in his possession.
Even more mysterious was the summoning process itself. Summoned beasts traverse space to arrive and fight for the user. They fight if they can win and flee if they cannot; instances of injury or life-threatening danger to them are exceedingly rare. This system of Summoning and Reverse Summoning—allowing entities to come and go via spatial teleportation so effortlessly even in a world with a generally stable spatial framework—filled Hiruko with doubt.
What concerned him even more was that the vast majority of practitioners must bite their fingers and use their blood as a medium when performing the Summoning Jutsu. Most research materials on the subject mentioned "using blood as a lead."
In the long ages before the rise of modern science in his previous world, many philosophers believed that blood was the carrier of the soul. Even in the thousands of years that followed, humanity failed to grasp what the soul truly was. Was its weight truly 21 grams, as some claimed?
However, in the shinobi world, whether there was a hidden catch to summoning and the use of blood remained a matter for debate. Who knew what these summoned beasts were truly trying to gain by demanding fresh blood?
Hiruko placed extreme importance on his own blood. Not only that, he even meticulously collected items like fallen hair. After all, once his DNA leaked, there were countless ways in the shinobi world to clone a perfect copy of him. He had no desire to cause major trouble for himself down the line, such as being exploited by someone with ill intent to create a "Great Villain" version of himself.
In the shinobi world, many ninjas use blood to assist in performing jutsu, but those who must use blood as a mandatory medium are few. As a part of the human body, blood is an excellent conductor of chakra. Generally, materials used for drawing Fuinjutsu (Sealing) formulas or performing special techniques often involve a mixture of ninja beast or even human blood.
There are also countless methods in the shinobi world that use blood to stimulate potential. The Three-Colored Pills of the Akimichi clan are classic examples of stimulating the body and overdrawing one's potential. Secret techniques that come at the cost of severely draining or burning one's own blood or body parts for a desperate fight are common. The logic that "only survivors get to reap the rewards" is a universal truth across all worlds. Dead is dead; no matter how incredible your potential, it's useless if you're gone.
Beyond that, there is Hidan's infamous technique from the Akatsuki: Curse Technique: Death Controller Possessing Blood. This is a bizarre and evil curse that kills others by using their own blood against them.
As for the underlying principle of the Summoning Jutsu, Hiruko wasn't entirely clear on whether it was linked to some form of sacrifice. However, he noted that the Aburame clan's Insect Technique involves exchanging their own chakra and offering their bodies as hives in return for the insects' help in battle.
This suggested that, unless the practitioner is dominant and the summon is extremely weak, it's hard to believe the Summoning Jutsu comes without a cost. Hiruko didn't believe for a second that high-level summoned beasts were so altruistic that they would help humans "grind mobs" for nothing.
Furthermore, high-level summoned beasts are intelligent species. Entities like Fukasaku and Shima of Mount Myoboku, or the main body of Katsuyu, are creatures standing at the pinnacle of power in this world. Without some kind of incentive, why would they interfere in human conflicts? Even if Kaguya Otsutsuki were resurrected and turned all of humanity into White Zetsu via the Infinite Tsukuyomi, what would that matter to them? Toads would still eat bugs, and snakes would still eat frogs. Why bother with prophecies or a "Child of Prophecy"?
If one were to name the most famous jutsu that requires a sacrifice to function, it would be the Reaper Death Seal (Shiki Fujin). Minato Namikaze, the Fourth Hokage, used this technique—offering his own soul—to summon an unidentified Shinigami to perform a soul-based attack and seal.
Literally translated, "Summoning" (口寄せ - Kuchiyose) implies a method of communicating with or calling upon certain existences. While its principles aren't identical to spiritual mediumship, the concepts share some overlap. The only thing Hiruko couldn't figure out was what the summoned beasts gained from humans, or what benefits they reaped from the contract. It was even possible they didn't gain from humans directly but instead benefited from the world's rules.
Without seeing the content of a formal contract, everything remained speculative. With this in mind, Hiruko once again scrutinized the data Minato had given him. Most of it focused on how to perform the jutsu, never touching upon what the jutsu was at its core.
At midnight, Hiruko suddenly realized that the detection devices he had placed around the village had been tripped. This was a coordinate-based "Hashirama Cell Detector" he had developed using Hashirama's cells. Because Hashirama's cells possess such massive vitality, they are easily detected; he had built this primarily to monitor that "filial son," Uchiha Obito.
Although there are quite a few people in the Leaf now who possess Wood Release—many clans had spent huge sums, with Minato's approval, to purchase the "Life No. 3" serum from Hiruko—this serum wasn't available to just anyone. Not every clan had the right to buy it.
Take the Uchiha clan, for example. Even though they were among the first to get involved, they were not eligible to purchase Life No. 3. As a Wood Release serum, the approval process was extremely strict; buyers had to be injected on the spot and recorded on file. Private reselling or identity fraud was strictly prohibited. Consequently, not a single vial had leaked; everything remained under Hiruko's tight control. Furthermore, every buyer had to have their genetic and chakra reactions recorded in his database after injection.
With these records in hand, using his devices to check for Uchiha Obito was as easy as flipping a hand. Minato had also used the approval power for Life No. 3 to reintegrate himself into the core of Konoha's power structure. The various ninja clans were delighted, spending a fortune to buy what they desired from the Fourth Hokage. After all, who wouldn't want the power of the First Hokage—a power capable of settling a chaotic world and bringing glory to one's clan?
But the most critical point remained Hiruko's lingering doubt.
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