Kakuzu could use Diamond Morph Jutsu to survive the explosion. He, however, didn't want to test out the power of something he never experienced before head on.
While chakra exhaustion was one thing, which has caused countless shinobi to fall on the battlefield, chakra overload was rather uncommon occurrence. And explosion from it was ever rarer. Just the fact that explosion was about to happen meant that abnormal amount of chakra had concentrated in a single point.
Also, just leaving the things in the room behind didn't sit well with him either. He was interested what scientist managed to accomplish and was sure some of it may be quite valuable. So, he wanted to find a way to avoid explosion burning down everything. And to do that, he had to somehow either confine the explosion in a small area or stabilize the source.
There were few ideas that came to Kakuzu's mind. One was to use seals to, well, seal the explosion. But since he wasn't proficient in the art, and didn't have enough time to do so, he scrapped the idea off.
Next was to use his threads to envelope Hiroshi's body and trap the explosion in the enclosed space. He then could apply diamond morph on it to further strengthen it. It would act as durable sphere that would dampen the impact. It seemed to be the perfect solution.
As Kakuzu was in the middle of finding how he would go about doing things, a reckless idea came to his mind. Something was drawing him towards the epicenter of it all, where the unstable chakra was coming from.
Why not?He thought to himself.
It wasn't really much of a risk since Diamond Morph would be enough defend himself… probably.
He approached Hiroshi. His attention was focused on scientist's ribcage, a place that was point where the chakra was being concentrated. Next moment, threads pierced Hiroshi's chest. And he started performing Reverse Earth Grudge Fear. Carefully, he infused chakra in scientist's heart, while simultaneously tried to keep the energy contained inside his body. It begun to somewhat work. Rapidly rising chakra volume begun to gradually slow down.
While stabilizing the chakra fluctuations, Kakuzu concentrated on his chest where his hearts were located. His back opened up and, as a precaution, he released one of the spirits from his body. It immediately left the room and started moving towards outside of hideout, where it would stay a safe distance away. In case that the explosion occurred, it would act as a sacrifice to revive himself.
Kakuzu concentrated. Now that he didn't need to worry about his survival, he could focus entirely on person in front of him.
"If this is going to work, it's going to open some exciting possibilities… So, let's see what we got here."
He extended one of the tentacles towards notebook on the table, he brought it to his side and then sealed it inside a scroll. He wondered if he could recreate the formula. He predicted that magnitude of explosion that was about to occur was about the same level, if not higher then Two Tail's Bijuu bomb. If he could somehow incorporate it into his arsenal, it would be of some help.
It took while for chakra fluctuations to stop increasing altogether and Kakuzu begun to further stabilize it by trapping and containing the chakra into Hiroshi's heart. Unlike with guards, however, this took much more time and effort to reach noticeable progress. He had to constantly pay attention for chakra to not loose control, or heart to not explode.
It took few hours for the familiar process to start unfolding before Kakuzu's eyes. First the scientist's skin, followed by muscles and bones begun to turn into the threads.
Hiroshi wasn't even a ninja, and his chakra reserves were next to nonexistent. So, reason and catalyst of the process taking this long should be the newly created potion. Kakuzu recalled that potion meant to increase the talent of human body. If that was the case, chakra system should be main target for the research. By stimulating it, potion could potentially enhance shinobi's chakra volume permanently. That was gist of it, but what really mattered was how Hiroshi went through achieving it.
A heart this weak shouldn't have that been able to emit that much chakra, and it shouldn't habe taken so much time for it to transform into the spirit. Kakuzu was curious about result. It made sense that since it took more time, the spirit should turn out to be stronger too.
As chakra fluctuation finally stabilized, a blob of tendrils was formed that was result of mass of threads merging and twisting around the heart. Kakuzu observed it patiently. He sensed that it had a lightning affinity that was rare outside of Kumogakure. Unfortunately, the heart didn't seem to be any stronger than it originally was.
Despite this, Kakuzu could feel something buried deep within this spirit. He took out a sealing scroll and summoned a mask with an yellow nose in the center. This was one of the Earth Grudge Fear masks, designed for a lighting affinity. But unlike previous ones that he used, this one was made by original Kakuzu.
"An [F-grade] huh... I expected more."
Kakuzu suppressed his disappointment. He just had to figure out what was so special about this one. He felt connection between this spirit to be much more clear than previous ones, which further convinced him to pay extra attention to it.
As he placed mask on top of the blob, the same process begun to repeat again. Kakuzu waited until spirit stabilized its form. But before he could better observe the spirit, something unexpected happened.
"…Hello?"
There was a voice coming out of the spirit. No, rather, Kakuzu was hearing the voice directly into his head.
"What happened to me boy? Is this your doing?" The voice repeated inside of his head again.
Communication was telepathic. It was similar to signals that other masks could send, but this was more advanced, it first time he heard coherent sentence through the connection. The other two spirits communicated with just intent and couldn't formulate words.
Kakuzu widened his eyes. A telepathy? Was this supposed to happen? The tone of the voice seemed familiar, but shouldn't that be impossible?
"Is that still you, Hiroshi?"
Hiroshi, or at least, entity in front of him that had the similar aura around him, looked down on it's body.
"Hmm...I'm not sure what I am anymore." His threads morphed into various shapes and finally stopped as they turned into claws. "I should be dead. Exploded to smithereens. Yet, here I am, alive, and looking like a monster from some horror story. What happened? How did you stop the explosion?"
"I used my jutsu on you to stabilize your chakra. It's also why you're looking like that. Though I wasn't expecting you to survive." Kakuzu said. "… tell me, how do you feel now?"
Hiroshi walked over to table where he was working on the potion and looked down at empty capsules, placed all over the place.
"Weird. It feels uncomfortable, and somewhat cold. Yet my body no longer aches and acts like a broken machine, so that's a plus. On top of that, my vision's near perfect, and I can hear much more clearly."
"So, um… Lad, what should I call you? You never told me your name. But looking at those tentacles, you're Kakuzu, right?"
"Yeah. That would do"
"You said you used your jutsu on me, but that doesn't explain much. Especially when there's no records of it left anywhere."
"Hmm… about that, you don't have to worry… I don't think I can do anything to that form of yours right now though. If you're concerned about the appearance just pick someone, and when I'll have time, I'll make you possess their body."
"That's not what I- actually… you can do that?"
"Yes. It should work, in theory."
"… In theory?! You kidding me? You never used it before?"
"Well, I never had a willing participant before."
And he didn't have the ability then, Kakuzu thought, but he would be keeping that to himself.
For Hiroshi, on the other hand, the phrase 'in theory' brought plethora of unpleasant memories.
"What about the other person? You're at least certain his consciousness won't affect me, right?"
Kakuzu was confident that it wouldn't be a problem. But scientist knew plenty enough, so he didn't answer, rather just smiled kindly to Hiroshi.
"Alright, keep your secrets, then." Hiroshi got the meaning behind it, so he chose to change the topic. "So, what's the plan? Where will you be going next?"
After observing a while, Kakuzu could deduce few things about anomaly standing in front of him. If normal spirits were like robots, this one would be akin to A.I. If original Hiroshi was dead or standing in front if him wasn't clear to him. But spirit was going to be staying under his wing. Unlike with regular spirits, he didn't have this one's full loyalty. Which was enough to warrant a concern. He wasn't sure he could trust him, or it for now. Granted, he could kill him anytime with a special trick.
"What about you doc? What are you planning to do?"
"Don't ask pointless questions lad. I can see the chakra pathway connecting us. I'm stuck with you." Hiroshi shook his head in annoyance. "I don't even know specifics of your jutsu. Do I look so reckless to you that you think I'll run away?"
'So, he can sense it too.'
The pathway of chakra that Hiroshi was talking abouy was what connected spirits to Kakuzu. The technique that Nagato used to control Six Paths of Pain would be close comparison to it. There were few differences between them, but the similarity was clear. Kakuzu could send chakra and instructions with it to the spirits. And the further the spirit was from him, more chakra it took to reach them. It was negligible for now, and even difficult to notice. But Kakuzu noticed that after he sent the spirits away, he needed more chakra to communicate with.
On the other hand, his chakra wasn't essential for them to function. Unlike Nagato, who had to constantly had to pump the chakra into the Pains unless he wanted for them to drop dead, the spirits produced their own chakra and had a certain autonomy. So, Kakuzu didn't need to divide his attention for them to fight, and if, for some reason, if he couldn't communicate with them, they would follow the previous orders they were given.
"You do seem that reckless old man… well, where was I? I'll figure something out. But, for noe, I'll be leaving."
"Oho? You're planning about visiting that island already? What is so important there that you're in such hurry?"
"Nothing much, yet. But I have someplace else to go before that."
"Is that so? Well then, before you go, I have a favor to ask of you."