Before he knew it, a year had passed since he'd arrived at Sakonji Urokodaki's cabin.
Once he'd confirmed that the magical particles in the air could also exist as True Qi, Dan Lei felt he'd discovered a wide, open road that the Type-Moon Magecraft world had never found.
The Breathing Technique passively dispersed True Qi through the flesh, strengthening the body enough to stand against demons. Dan Lei, however, had chosen to build up his strength to achieve Total Concentration Constant.
The outer layer of the meridians was made up of Magic Circuits. He only needed to use those Circuits to seal the True Qi inside his meridians. Even if it leaked, it could only spread within them. When more True Qi flowed in afterward, it would naturally push the True Qi ahead of it through the body.
Then, once his meridians were truly full, his Magic Circuits could draw out some of the True Qi. That completed the continuous flow through his meridians, without any risk of rupturing them.
Finally, the mana refined a second time by his Magic Circuits flowed back into his body, and Dan Lei felt his physical abilities begin to improve rapidly.
Moreover, once his meridians had been strengthened to a certain degree, he could also reverse the process he'd initially envisioned. As long as the True Qi didn't make his blood flow backward or drain the energy from his flesh, nothing would go wrong.
In fact, if Dan Lei had a professional Divine Era Magician nearby, they would have told him that he was just like a young divine beast or magical beast entering a growth phase: his body was absorbing energy and strengthening itself every day.
As for Tanjiro, Kojiro Sasaki gave him occasional guidance on swordsmanship, and his skills improved faster than in the original story.
In the original story, it took Tanjiro a year to learn every Water Breathing form. This time, after ten months, Sakonji Urokodaki had already said he had nothing left to teach him and told him to go cut a boulder.
But this time, Tanjiro didn't have to practice swordsmanship alone. Dan Lei and Kojiro Sasaki took turns training with him.
More or less, the pattern was that Kojiro Sasaki would beat Dan Lei at swordsmanship, after which Dan Lei would go to Tanjiro to regain his confidence.
Sometimes, when Kojiro Sasaki saw Tanjiro hit a wall in his swordsmanship, he would step in himself and help him break through it.
And by "help him break through," that meant literally breaking him through it. They sparred the whole time, making Tanjiro experience his weaknesses firsthand.
As long as he got enough nutrition, those minor flesh wounds were no problem—Dan Lei could treat them.
The worst time poor Tanjiro came to Dan Lei for treatment, he had at least two hundred wounds on his body. He was covered in blood, and the experience even helped Dan Lei's power of the Abundance Destiny advance. It also inspired his first technique for treating others: Medicine Dragon Healing the World.
Well, the move was just an imitation of Azure Dragon Cleanses the World. He made a Water Dragon filled with the healing power of the Abundance. It could barely hurt a fly, but toss it onto an injured person and let it circle them a few times, and it would heal them quickly.
Of course, Dan Lei and Kojiro Sasaki treated Tanjiro sincerely, and he was very grateful to them.
Though entirely by accident, during the New Year festivities, Tanjiro went outside on his own and performed all twelve dances of his family's Hinokami Kagura.
That saved Dan Lei the trouble of coaxing him into teaching Sun Breathing. With his current memory, Dan Lei had memorized all twelve dances after seeing them just once.
So this time, Tanjiro didn't need half a year. Just three months after Sakonji Urokodaki set him the task of cutting a boulder, Tanjiro split it with a single strike.
However, splitting the boulder early didn't let Tanjiro join the Demon Slayer Corps ahead of schedule. The Demon Slayer Corps' Final Selection wouldn't be moved up just because he'd progressed faster in his training. He still had to wait until the time set in the original story.
Still, Dan Lei said his farewells once Tanjiro had finished the boulder-cutting task.
Actually, Dan Lei had completed Total Concentration Constant before Tanjiro had even learned all of Water Breathing. And he hadn't taken any shortcuts; he'd truly done it using only the Breathing Technique.
Dan Lei's rapid progress was tied to how quickly his body was growing.
At this point, Kojiro Sasaki judged that Dan Lei had already surpassed Giyu Tomioka in everything except swordsmanship experience and adaptability in battle.
According to Kojiro Sasaki, the only way Dan Lei could further improve his swordsmanship now was through real combat—not sparring with Kojiro Sasaki, where he knew he wouldn't die, but facing an enemy's killing intent head-on, in a fight where they stood face-to-face and every strike drew blood.
As for Dan Lei's research into demons, that had also reached a bottleneck. He couldn't imitate Blood Demon Arts, and the bloodlines of ordinary demons were too muddled. He'd made no progress at all in deciphering the Oni Clan's bloodline.
His research into absorbing energy through Ley Line abilities had also stalled completely, because Nezuko had absorbed enough energy and fallen into a deep sleep.
Still, wanting to see things through to the end, Dan Lei stayed until Tanjiro had completed all his training before saying goodbye to Sakonji Urokodaki and Tanjiro.
The Demon Slayer Corps hadn't contacted Dan Lei once during the past year or so. Clearly, they still regarded him with considerable suspicion.
But that was understandable. If the Demon Slayer Corps weren't cautious, Kibutsuji Muzan would have wiped them out long ago. They wouldn't have been able to fight him for so many years.
Fortunately, Dan Lei wasn't determined to meet the Demon Slayer Corps' leader. He had no mission in this world. His Holy Grail's energy just wasn't full yet, leaving him stuck here.
So Dan Lei was entirely free to do as he pleased. It was just that the way he'd been raised in both his previous life and the one before that had made him someone who was willing to follow rules and order. When they seriously clashed with what he wanted, he followed his own desires; when the conflict wasn't serious, he followed the rules.
However, thanks to the influence of his second life, no one should expect Dan Lei to put ordinary people's safety first once he started acting.
He would take ordinary people's safety into consideration, but if looking after them seemed too much trouble, he would simply ignore them.
And if ordinary people really got in his way out of ignorance, Dan Lei was perfectly capable of killing and setting fires.
If he had to classify himself according to D&D alignments, Dan Lei thought he would be Neutral Evil, though leaning toward True Neutral.
Tanjiro was reluctant to see Dan Lei and Kojiro Sasaki leave, but he knew they couldn't stay with him forever. In the end, he said goodbye through his tears.
As for Sakonji Urokodaki, he'd seen enough partings to keep his emotions in check.
Before leaving, though, Dan Lei asked Sakonji Urokodaki for two Nichirin Blades.
Nichirin Blades could kill demons because the metal used to forge them absorbed some kind of solar radiation, which was a natural enemy of demons.
As for why demons could only be killed by cutting off their heads, Dan Lei had dissected dozens of them and still hadn't found the reason. All he could say was that it was a rule set by the Creator.
But if anyone expected Dan Lei to imitate that radiation, they had too much faith in him.
It might have looked like he knew what he was doing when he studied demons, but nothing about them had ever gone beyond the bounds of his knowledge.
Dan Lei's knowledge mainly came from three sources: his understanding of the settings of various worlds from the life before his previous one; the knowledge related to automaton crafting that he'd studied under Aozaki Touko in his previous life; and, in this life, the basic scientific and medical knowledge of the Luofu, knowledge of mechanical puppets, the basic principles of Pathstriders, and the ordinary Cloud Chant Technique that members of the Chizuru Clan could learn.
But none of that covered Stellar Energy. Recreating the fluctuations of solar radiation was a field of research for the Department of Astrology at the Clock Tower, and on the Luofu it was studied only by specialists.
Dan Lei belonged to neither side, so naturally he had no idea how to reproduce something like stellar radiation.
So even though he had ways to kill demons, cutting off their heads with a Nichirin Blade was still the most efficient method.
Dan Lei wasn't foolish enough to ignore an easy solution and insist on using his Magecraft to kill demons.
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