It was 100 or so years after the Battle with Muzan all the slayers we know and love now in the afterlife all but one from that time is still around. Yushiro a demon boy created by Lady Tamayo is at his home painting portraits of her so that he can remember her forever. He wanted to end it all but Tanjiro told him that he should love on to keep her memory alive.

Yushiro decided to take a break with the new masterpiece he was working on and turned on the TV for some world news that might be "important" for him to know. As he was about to turn off the TV the news anchor said something that will shock him to his bones.

"BREAKING NEWS! THERE HAVE BEEN REPORTS OF A BABY IN CHINA THAT HAS DEVELOPED SOME SORT OF SUPERPOWERS! HERE IS KATSUME AT THE HOSPITAL WHERE THE BABY WAS BORN."

"This is just some of that fake bullcrap that's been popping up lately, this doesn't feel any special." He scoffed at the TV. He tried flipping through other channels hoping something more interesting was on but found nothing. "Well this was a waste of time..." Yushiro grumbled then he turned to the unfinished painting and smiled softly. "Lady Tamyao I swear that today I will finish that beautiful face of yours..." then he got up from his seat with his determined face and started working on his art piece.

Over the next few years, more babies were born with different powers and by this point, Yushiro was now confused and starting to get worried.

More and more children are being born these powers? Where did these powers come from? Why now are these powers showing up? None of these questions were being answered just the same answer again and again."It's the next evolution of mankind!"

"What the hell? I keep hearing on and on about this, now it's been reporting about these strange births all over the world! Could this be something of Muzan?" Yushiro questioned himself, he thought about it for a moment then shook his head. "No! What the fuck am I saying!? He's been dead for over a century there's no way in hell that he's behind this!" He yelled in frustration.

"Meow?" the sound of Chachamaru as if it was asking Yushiro if he was okay.

Yushiro took some deep breaths to calm himself down and smiled at the Calico Demon.

"I'm okay Chachamaru, I'm just confused about these "Quirks" as they call it... They closely resemble the blood demon arts that demons had, I'm afraid that these "Quirks" are a scheme to bring the demon race back to existence..." He mumbled to the cat. He didn't want to believe it, but the thought never seemed to leave his mind, so he reluctantly decided to withhold his paintings and dedicate his entire time researching quirks to find the real "truth" behind their origins.

So over the next 200 years he research's these powers now called "Quirks" learning some valuable information about them. What he came across however shook him to his very core. These powers were never with humans all at, it was originally from Rats and Mice that were "infected" from an unknown source. Yushiro immediately started panicking at the dark truth.

"No... it can't be true! This has to be a lie!" He shouted out in disbelief. "Damn it, Damn It, Damn it! I thought that they were all killed after the fight with Muzan! Did one survive and now it wants to take revenge!?"

He looked through all of the notes that he wrote down in the various journals that he "borrowed" over the centuries and came to a very dark conclusion.

These "Quirks" are Blood Demon Arts that have mixed with Human genetics which means...

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Demons like him could still be alive and want their revenge.

Yushiro clenched his fist in anger. For centuries he has isolated himself from society doing nothing but paint and taking care of Chachamaru, not once has he had made contact with Tanjiro or any other slayer that he knew from the big fight against Muzan all those years ago. He completely left them in the dust, almost forgetting them due to his grief over Tamayo's death. Now there is no one alive that still remembers him at all, nor has heard anything about demons, if what he thinks is true then it wouldn't be Japan that would be in danger but the whole world!

Yushiro knew that he couldn't just sit around and let the world burn around him, so he wasted no time packing some basic needs that he and Chachamaru would need for their journey. The only people that might know something about him are the Kamado and Ubuyashiki families however he had one major problem.

He had no fucking idea where to find them

Yushiro luckily got something of a headstart due to him finding an article made around 205 years ago about Kiriya Ubuyashiki being the oldest living person in Japan at that time, it said that he was living in Kyoto at the time of release so Yushiro decided to start there.

"Alright Chachamaru, you ready to go? We got no time to lose." Yushiro asked the calico who hopped in his basket to avoid being covered by the sun. As Yushiro took a step outside he stopped and turned back into the house to the last painting of Tamayo that he worked on before he started his quirk research.

"Lady Tamayo, I swear that me and Chachamaru will find out who's doing this and put a stop to their plans. I promise that I won't fail you again..." He told the painting, he placed her hairpin that he carried with him for all these years on the frame and headed out to begin their new journey.