Falling Despair

Entering the Infinity Castle, Isao marveled at its unique architecture, the endless extra-dimensional space of wooden rooms, the endless halls, and the moving corridors. 

This was a boundless, infinite space! 

Watching all of this, Isao longed for this place to become his, so even if Muzan was by his side, he didn't care and charged forward toward the castle's owner. 

Muzan's heart skipped a beat as Isao burst into the room. He never expected Isao's reaction to be so fast, and Isao could enter this place just a moment before the door of the portal closed. 

Preparing for battle while running away as he called for his followers, Muzan was taken aback when Isao seemed to disregard him entirely, turning on his heel and fleeing from the room in a flash. 

'Is he afraid?' 

No.

As someone who was almost killed by Isao, Muzan knew how rude this guy was, and probably a fear never had been on Isao's head, so if there was something that Isao tried to do, then it would be this place! 

Isao was going to kill Biwa Demon! 

Muzan knew that he had to stop Isao no matter what. If Biwa Demon died, no one could control this place anymore, and there was a chance they might be trapped or thrown out of Infinity Castle. 

When that happened, Muzan would be like a fish pulled out from the water and waiting to be chopped off by the chef! 

'No! I can't let him succeed!' 

Even if many of them died, only he and the Biwa Demon had to live! 

The two had to live no matter what, for as long as they lived, there would be a chance for revenge in the future! 

A human's life was limited, and sooner or later, Isao would die. By then, Muzan was going to swallow this world into an abyss! 

Unfortunately, when Muzan was about to control the Biwa Demon, he saw many flowers appear before him, and sweet scents were smelled by his nose, causing him to be in a daze as if he had fallen into a hallucination. 

"Sorry, you will die here, Muzan." 

Taking out a serum from her pocket, Tamayo then cruelly injected a potent poison into Muzan's neck. Even if Muzan was wounded and Isao cut half of his body, she continuously attacked Muzan's head, trying to kill him, but she knew that with her power, no matter what she did, it was impossible to kill Muzan. 

So, all she had to do was to buy time for Isao. 

As long as Isao could take down Biwa Demon, then Muzan was as good as dying. 

'I can't buy time for so long, so you have to be fast, Isao-kun...' 

Her eyes moved in the direction where Isao had disappeared, and she prayed for his success. 

Still, before that, she was going to torment Muzan as long as she could and, at the same time, try to slow down his recovery. 

At the same time, even with how strange the space was, Isao didn't seem to be confused and moved so smoothly without stopping, letting his nose lead him to the owner of this Infinity Castle. He moved like a centipede that moved around the gap of a stranger's house without being noticed by anyone, and when the owner realized it was already too late, he had already entered and was ready to snap off the head of the owner with its mandibles. 

Still, unlike before, Isao knew that all of the demons that were present weren't good anymore. Someone like Akaza was rare, and the rest of the demons became a demon because of how demonic their personality was. 

In other words, killing them did a good thing for the world. 

This also made him quite sentimental somehow. After all, when he came to this world, he was almost killed, then rejected the invitation of the Demon Slayer Corps, antagonized many people, and slept with the protagonist's mother; then, at the end of the day, he became a hero who would slay the demon that had terrorized this country for a millennium. 

While his stay might not be as long as when he was in the world of "High School of the Dead," he had done many things in this world, and he was also about to erase all the demons before even the protagonist finished his training. 

Nevertheless, after getting sentimental enough, it was time for him to take down Biwa Demon. 

He was about to reach his target, but as expected, Muzan recovered and told all the demons in the Infinity Castle to stop Isao. 

"Stop him!" 

Watching this, Tamayo quickly took out a rapier-like Nichirin Katana and stabbed it into Muzan's head, but it was too late since all the demons had been remembered, and they were all on the move to stop Isao. 

While only some of the demons knew about the existence of the Infinity Castle, Muzan had kept many mindless demons who couldn't transform properly and moved like animals within this place so they could be used as goons to surround those who entered. 

Moreover, meeting their opponent directly wasn't suited to their status, so getting goons was something necessary, especially when the number was also part of the strength. 

Yet Muzan didn't care about those goons since he knew they couldn't stop Isao. Instead, he ordered the Twelve Kizuki, who had stayed on the Infinity Castle, to stop Isao no matter what! 

While they were surprised by Muzan's order, they quickly made a move since they didn't want to be killed by Muzan, so they made their move quickly, yet they were dumbfounded when they saw the scene before them. 

Many of those failed demons were killed, and no one could stop the movement of this Demon Slayer. 

Isao's movement was like a dung beetle that rolled over the enormous dung ball without stopping. 

Or rather, it was impossible to stop him! 

Yet, Kokushibo, the Upper Rank 1, quickly realized the identity of Isao, and he knew that this should be the reason why Muzan had been in a bad mood. At the same time, this young man should be the person who killed Doma, Akaza, and many others. Realizing Isao's identity, he had no hesitation in himself, and he crafted a katana with his own body as he was ready to face Isao. 

"Moon Breathing..." 

Still, as Kokushibo underestimated Isao, thinking that Isao was an existence below his little brother, suddenly, his body stopped and couldn't move. 

'Huh?' 

Kokushibo was confused about why he suddenly lost control over his body, yet Isao didn't give him a chance.

With the hammer part of his poleaxe, Isao smashed the strongest demon under Muzan, breaking Kokushibo into pieces of meat. 

"..." Everyone. 

[You have acquired the Kokushibo's DNA.]

[Do you want to absorb it?]

Isao absorbed everything about Kokushibo, killing him by smashing his hammer so many times, causing all the demons in the present to be stunned still. 

Whether it was Biwa Demon, the members of the Twelve Kizuki, or those failed demons, all of them could only stun silly while watching Isao demolish Kokushibo, the strongest demon under Muzan. 

Gaining Kokushibo's abilities, memory, experiences, and everything else from his DNA, Isao then focused on Biwa Demon, a female demon who transformed into a demon because she had a hobby of smashing someone's head with a hammer before her performance as a biwa player. 

"Goodbye." 

Biwa Demon thought of running away quickly, using her ability to escape so far away on the distance part of the Infinity Castle, then threw Isao away, but when her brain just started to think of a plan to run away, Isao was already by her side and slashed her body into half. 

"..." 

[You have acquired Biwa Demon's DNA.]

[Do you want to absorb it?]

Finally! 

Yet, watching how all of those powerful demons were killed by Isao, all of the demons present felt intense despair that was about to pull them into a bottomless abyss.