Across the ninja world, many shinobi use blood to assist in performing techniques, but very few ninjutsu require fresh blood as an absolute medium.
Blood, being part of one's body, serves as an excellent chakra-conducting material. Beast or even human blood is commonly mixed into materials used for drawing Sealing Techniques, Barriers, and ritual arrays. Mizuki often uses his own blood to strengthen control when performing Kuchiyose no Jutsu because water jellies have extremely low intelligence and are difficult to manage effectively.
Another typical type involves techniques that cause chakra overuse, pushing the body beyond its limits. Secret arts that burn one's life force for desperation techniques are quite common.
Beyond these methods exists a further category: sacrificial techniques. The most infamous example comes from Akatsuki member Hidan, whose [Death under Pledge] technique—a bizarre and sinister curse—requires the sacrifice of either one's own or another's blood.
As for the principles behind Kuchiyose no Jutsu, Mizuki wasn't sure whether there were any similarities with such sacrificial arts. However, looking at the Aburame Clan's Bug-Manipulation Art, it clearly functions by exchanging personal chakra—using one's own body as a hive to allow Bugs to fight on one's behalf.
Therefore, in general, unless it's a case like Mizuki's where the summoned beast is significantly weaker and he remains in full control, it's hard to believe summoning techniques truly carry no cost whatsoever yet still bring powerful beasts to aid in battle.
Moreover, higher-ranked summoned beasts are intelligent beings. Outstanding examples like [Great Toad Sage], [Snake Sage], and [Katsuyu] represent some of the strongest creatures in this world. Without any personal stake, why would they involve themselves in conflicts among humans? Even if Kaguya Otsutsuki were reborn, enslaving all humanity through [Infinite Tsukuyomi] and turning everyone into White Zetsu, what would it matter to them? Frogs would keep eating insects; great snakes would continue feasting on frogs. Why go out of their way to talk about predicting a "destined child"?
If any technique famous for demanding a serious sacrifice had to be named, it would definitely be [Dead Demon Consuming Seal]. This technique required sacrificing one's soul to summon a mysterious entity known as Shinigami, capable of launching soul attacks and performing sealing techniques.
Summoning techniques, as the name suggests, can be understood as a form of communication with certain existences. Though not in the same category as [Dead Demon Consuming Seal], they did share similar underlying principles. The only mystery was—what exactly did summoned beasts gain from humans?
Or...
[Perhaps they don't gain anything directly from humans, but instead benefit indirectly from changes within the entire world. If so, then it could make sense. Still, without seeing the actual contract scroll, it's impossible to say for sure.]
Thinking thus, Mizuki picked up the Summoning Technique materials again and read for a while, though he found nothing useful.
[Looks like I'll need to conduct a proper investigation before reaching conclusions.]
...
The next morning, Mizuki checked on the counterfeit jinchuriki who was still recovering from injuries. His condition had stabilized and was no longer in danger. Actually, during this escape attempt, Mizuki had tested a violent version of space-time ninjutsu purely out of necessity.
Originally, Kuchiyose no Jutsu worked perfectly before he became the artificial Jinchuriki. But now, after a thorough check, the reason became clear — Kuchiyose no Jutsu is a technique that connects two separate individuals, yet the summoning contract now identifies both his original self and the artificial Jinchuriki as the same person, rendering regular summoning ineffective. At most, he could only use certain methods to enhance the sensory connection between the two. Otherwise, Mizuki wouldn't have been so eager to develop [Reverse Summoning]-based spatial ninjutsu. However, he truly hadn't expected real space ninjutsu to be so difficult — it completely exceeded his expectations, which was why he had become so focused on techniques like [Kuchiyose no Jutsu] that enabled Space-Time transmission. Only now did Mizuki truly appreciate how powerful [Flying Thunder God Technique] was — a space-time jutsu developed based upon Reverse Summoning.
Inside Konoha Hospital, many injured Genin exam candidates were receiving treatment. In fact, since Mizuki currently had no active missions, few people came to bother him. As he walked through the ward, a masked face could occasionally be seen among the corpses being carried out — some of whom had been personally saved by Mizuki himself.
Arriving at the top-floor VIP ward, Kakashi was already waiting there.
"What do you need me for, Kakashi?" Mizuki asked as he entered, quickly scanning the room and spotting several Anbu stationed around.
"You've seen it yourself — Sasuke's injuries are severe," Kakashi gestured. "But he must participate in the third round of the Chunin Exams one month from now. I need to give him some special training, which also serves as protection."
"And?" Mizuki asked, puzzled. "You didn't call me here just to show off your teacher-student bond, right?"
"Hear me out first," Kakashi patiently explained. "Protecting and training Sasuke alone will take a significant amount of time, yet all three of my students made it into the final exam."
"Oh right, almost forgot to congratulate you, Mr. Leaf Technician," Mizuki remarked sarcastically. "Great job mentoring!"
"Erm... let's skip that part," Kakashi replied with slight embarrassment, actually admitting that aside from teaching basic skills, he hadn't given much proper instruction to these Genin. "Therefore, my other two students temporarily lack guidance. Targeted improvement before the final round of the Chunin Exam is very necessary, so I asked Ebisu to tutor Naruto, who has the weakest foundation."
"Ah, now I get it," Mizuki nodded. "So you want *me* to help train Sakura?"
"Precisely. Please, do help me."
"You must've misunderstood," Mizuki immediately replied. "Ebisu specializes in teaching — it's understandable to ask him. But why come to me? Don't tell me everyone's right about you playing favorites."
"Don't refuse so quickly," Kakashi defended. "During last time's preliminary exam, Sakura figured out it was a village test, so she asked me who the examiner was."
"So you sold me out?"
"That's not really betrayal," Kakashi reasoned. "It was actually Sakura's own request to have you guide her."
"Really?" Now this was surprising. "What use would she have for me? Her opponent is that strongest Otogakure ninja — the gap is too big. Does she have something else in mind?"
"To be honest, I don't get girls' logic either. But we really couldn't find anyone more suitable to teach Sakura."
"So you're dragging me in just to make up the numbers?"
"Well, not really. Considering your strength, techniques like Sealing Technique, Genjutsu, or Barrier Technique—Sakura can't learn anything beyond a basic level right now. There's not much difference between your teaching her and mine. As for other things, her power can't be improved quickly anyway."
"Sealing and Genjutsu barely count as useful. What's the point of learning barriers? She'd be better off resting and saving her energy. The opponent's sound wave attack was really troublesome."
"Anyway, I'll leave it to you. Let Sakura learn whatever she wants. It's up to you."
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