His lungs no longer drew powerful breaths, and his muscles no longer had the resilience a swordsman should possess.
He had regressed from a Demon Slayer Corps swordsman into an ordinary person.
But demon blood had already entered his body. It was Kibutsuji Muzan's blood, and an ordinary human body simply could not withstand such a forced transformation.
Lump after lump bulged beneath Kaigaku's skin, and his bones made twisting, snapping sounds.
His limbs swelled rapidly, his chest and abdomen expanded like an inflated ball, and his veins bulged one after another as his entire body warped completely out of control.
"Help... help me..."
Kaigaku's voice cut off abruptly.
After swelling to its limit, his body finally burst into a mass of flesh and blood.
The instant his flesh exploded, Kokushibo had already darted more than a dozen paces away.
Scarlet scraps of flesh and blood splattered across the ground. Beside the corpses of several Demon Slayer Corps swordsmen lay another mass of remains, twisted beyond any recognizable human shape.
Kaigaku was dead, and his death had been ugly.
All six of Kokushibo's eyes turned toward the mass of flesh and blood. There was no pity on his face, only indifference.
"Trash."
He spoke in a low voice.
He had been given Lord Muzan's blood, yet he lacked even the qualifications to withstand it.
Just moments ago, when Kaigaku had knelt on the ground begging for his life, he had sounded more resolute than anyone.
"I can withstand it! I know I can!"
Yet barely a moment after the demon blood entered his body, he had exploded.
Kokushibo had originally thought this swordsman who used Thunder Breathing had at least some value worth cultivating.
Now it seemed he amounted to nothing more than this.
But soon, Kokushibo's gaze fell on the scroll in his hand. He had heard Kaigaku's screams before his death with perfect clarity.
"Where's my strength?"
"My body... why..."
This was not simply a failure to withstand demon blood. It was more as though something had drained away his cultivation and physical foundation before the demon blood could transform him.
And this scroll had appeared at far too convenient a moment.
The moment the demon blood fell into Kaigaku's mouth, the scroll had dropped out of thin air.
Kokushibo lowered his head and slowly unrolled the scroll. He could read the words on it, but the contents were utterly unfamiliar.
"Lightning Release: Lightning Ball Technique."
"Chakra."
"Hand seals."
Kokushibo read on word by word, the scrutiny in his six eyes deepening.
Lightning?
This was different from Thunder Breathing.
Thunder Breathing merely strengthened the body through special breathing methods, then used sword techniques to imitate the speed and might of thunder.
But what was written in the scroll seemed to involve actually converting some kind of energy into lightning.
He had never heard the word chakra before.
Kokushibo raised one hand and followed the scroll's instructions, attempting to mobilize the power within his body.
There was no response.
He then awkwardly shifted his fingers according to the illustrations.
Still, no lightning appeared.
All that flowed within him was the power of demon blood, along with the swordsman's instincts honed through Moon Breathing.
For him, this so-called chakra did not exist.
Kokushibo stopped and rolled the scroll back up.
"An object from another world?"
He did not waste any more time.
Something that could cause such a change in Kaigaku when demon blood entered his body was certainly no ordinary object.
Just because he could not understand it did not mean Lord Muzan would fail to see what was wrong with it.
Kokushibo tucked the scroll into his sleeve and cast one last glance at the mass of flesh and blood on the ground.
"Kaigaku, you weren't even qualified to become a demon."
As his words fell, his figure flickered and vanished into the depths of the mountain forest.
Fang Xu sat at the table and finished watching the entire exchange recording.
After the images faded, he remained silent for a long while.
Six eyes. Upper Rank One. Kokushibo.
And the boy kneeling on the ground, clinging to his leg and begging for mercy-Kaigaku.
He had recognized them.
"The Demon Slayer Corps' world."
Fang Xu let out a breath, a somewhat strange expression on his face.
Kaigaku had originally been Agatsuma Zenitsu's senior fellow disciple. He had learned Thunder Breathing, but could only use the forms other than the First Form.
Later, he had betrayed the Demon Slayer Corps and defected to Kibutsuji Muzan. After becoming a demon, he had even become an Upper Rank.
But this time, the exchange had changed his fate outright. The demon blood had barely entered his mouth before he exploded.
Fang Xu did not feel the slightest guilt. He even felt a little satisfaction.
For someone who betrayed his school, caused his master's death, and looked down on Zenitsu, death was death. The more miserable it was, the better.
"One scroll of the Lightning Ball Technique to screw a future Upper Rank to death."
Fang Xu could not help chuckling.
"Not a bad trade."
Just then, a speck of blue light suddenly appeared on the tabletop. It rapidly gathered, turning into a thumb-sized crystal.
The crystal was deep blue throughout, with fine lightning patterns moving inside it like a sealed mass of lightning.
The next moment, notifications appeared in Fang Xu's mind.
[Exchange complete.] [Obtained: Thunder Breathing Crystal.] [Notice: This crystal contains Kaigaku's experience cultivating Thunder Breathing, his swordsmanship experience, and part of the physique and cultivation he developed over many years of training.] [Notice: Because the exchange extracted Kaigaku's cultivation and physique as a swordsman, his flesh could not withstand the demon blood's transformation and ultimately collapsed.]
As Fang Xu looked at the blue crystal, his breathing grew noticeably heavier.
So that was it. Kaigaku had not simply been unlucky. His Thunder Breathing cultivation and swordsman's physique had been directly extracted through this exchange.
If an ordinary person suddenly swallowed Muzan's demon blood, it would have been stranger if he had not exploded.
Fang Xu reached out and picked up the crystal. It was cold, yet it seemed to hold a tingling electric current within.
The instructions were simple: swallow it to absorb it.
Fang Xu did not hesitate. What he lacked most right now was something that could immediately fill out his combat system.
Thunder Breathing suited him perfectly. The Rumble-Rumble Fruit produced real lightning, and Lightning Release Chakra was real lightning too.
Thunder Breathing used the human body, breathing, footwork, and sword techniques to imitate lightning. If these three could be combined, the improvement would definitely be considerable.
Fang Xu placed the crystal in his mouth.
The next second, it rapidly melted.
A cold yet searing power rushed down his throat into his body, then exploded through every limb and bone.
Fang Xu's body shuddered. His muscles, bones, lungs, and heart all felt as though a force were washing through them anew.
A vast amount of memories also flooded his mind.
Thunder Breathing's breathing rhythms, the application of force in its swordsmanship, and the variations in its footwork.
Second Form: Rice Spirit, Third Form: Thunder Swarm, Fourth Form: Distant Thunder, Fifth Form: Heat Lightning, and Sixth Form: Rumble and Flash.
There was also Kaigaku's training experience from countless instances of swinging his sword, running, breathing, getting injured, and making adjustments.
These things were not simply being stuffed into his mind. It was more as though they were being engraved directly into his body.
Fang Xu could feel that his breathing had changed.
Every breath his lungs drew could engage his muscles and bring forth strength.
His legs felt as though they had been tempered countless times. Their explosive power, stability, and coordination all rose sharply.
The feeling in his wrist as he held a blade was also completely different from before.
If he had previously been only a novice whom Hatake Sakumo had led across the threshold of kendo, then now he had gained a seasoned swordsman's combat instincts out of nowhere.
Of course, this did not mean his attainment in kendo had surpassed Hatake Sakumo's.
Thunder Breathing belonged to the Demon Slayer Corps' system.
The Hatake Sword Technique was kendo from the Shinobi World.
The two were not the same thing.