The new demon king (Completed)

The past

Narrator pov:

15 years after muzans defeat. Still in the Taisho era

The demon slayers were broken, with all the hashiras dying and with the new demon king, stronger than ever, the demon slayers needed to work harder than before. But at this time they were still reforming the hashiras. Former hashiras were given the order to immediately begin training the demon slayers to become stronger. The son of Kagaya Ubuyashiki, Kiriya. He was still so young yet he already had to lead the demon slayers, he didn't want his father, his mother, and his sister's sacrifices to go in vein. With demon slayers spread around the country thin, it didn't take too long until Tanjiro made a move.

Tanjiro was walking through the center of a village that was on fire with his katana by his side, there were screams, there were dead bodies and blood lying everywhere. As tanjiro was walking forward he stopped as he saw 5 demon slayers arrived, these demon slayers were slayers that were almost as strong as a hashira, their ranks were kinoe. They each wore a different colored haori except for two slayers. Two of the slayers wore the same cyan haori and both with the same blonde hair they were both twins, a slayer who was female had short black hair but she wore a bright purple haori, another slayer who had brown hair and wore an orange haori, and the one leading them was wearing a striped brown and yellow haori but he had black spiky hair.

They stood across Tanjiro while Tanjiro only put on a small smile before gesturing them to come at him. The slayer that was leading them immediately got into a stance preparing to do a form. He said before unleashing the form "breath of thunder Third Form: Thunder Swarm" The user surrounds the enemy with waves of arched lightning and attacks them from all directions, but he didn't even land a scratch on Tanjiro, the second he was an inch away from him getting ready to decapitate him, Tanjiro punched him back so hard that he went flying back to the spot where he stood with his squad, creating a small crater and the spot where tanjrio punched him left a hole on his chest as he immediately died.

The slayers stood and watched in fear as the saw Tanjiro's fist with blood, Tanjiro looked at the other slayers as they also got ready to attack. The twinned slayers immediately got into the exact same stance and muttered "breath of water, tenth form: constant flux" The users performs a continuous flowing attack which seemingly takes the form of a water dragon that increases in strength and power with each rotation due to momentum. They both were attacking Tanjiro using the same form at the exact same time at both the left and right. Tanjiro only stood still and watched as he placed his hand on his katana. The second the two twins got close, Tanjiro unleashed his sword with one quick slash, the twins were now behind tanjiro, it seemed like they didn't do damage to each other until both twins chest spewed out a lot of blood. Tanjiro had slashed them but so fast they didn't even notice until their chests spewed out a lot of blood as both twins immediately died.

The last two kinoe slayers were staring in fear, they both looked at each other and nodded before looking back at Tanjiro and charged but not before they each muttered their own breath, the male muttered "breath of fire, fifth form: flame tiger" The user unleashes a powerful series of sword slashes that seemingly takes the form of a rampaging tiger engulfed in flames that charges towards the enemy at blinding speeds. While the female muttered "breath of serpent, Fourth Form: Twin-Headed Reptile" The user leaps forward and performs a horizontal slash that slices through the target.
With the flame user reaching Tanjiro first, Tanjiro slashed from the left waist up to his right shoulder slicing him to two. While the female who just reach him was immediately decapitated by Tanjiro.

While Tanjiro stared at the corpse of the kinoe ranked slayers. He looks back and sees an old friend, "Zenitsu..." Tanjiro said while Zenitsu stared with anger and with sadness seeing what his friend had become "Tanjiro.... Why? What happened to you?" Zenitsu asked with tears flowing down from his eyes while Tanjiro only stared and smiled "why not become a demon? I need to replace the twelve demon moons..." Tanjiro said " And I can't think of a better person than the man who created the seventh form of breath of thunder" this made Zenitsu even more angry, Zenitsu immediately got into position and muttered "breath of thunder, first form: thunder clap and flash! GOD SPEED" An extension of the Thunderclap and Flash created by Zenitsu and the fastest version of the technique. The user performs Thunderclap and Flash while greatly augmenting the speed of the technique.

But Tanjiro only stood still and drew out his boney tentacles in a millisecond and deflected back Zenitsu's attack and forcing Zenitsu to go back, suddenly Zenitsu coughed up blood while Tanjiro's tentacle was chopped off as Tanjiro retracted his tentacle. "Zenitsu... it's not too late... come become a demon" Tanjiro said but the area around Zenitsu suddenly changed as Zenitsu muttered "breath of thunder, seventh form: Flaming thunder god!" The personal creation of Zenitsu Agatsuma. The user dashes forward at blinding speeds and unleashes a singular forward slash that is seemingly accompanied by a yellow Japanese dragon shaped like a lightning bolt. The form actually hit Tanjiro and chopped his arms off but they healed in a millisecond and Tanjiro punched through Zenitsu's stomach. Tanjiro now with his arm through Zenitsu's stomach and standing as tall as he is said "sorry old friend... but it's time you rest and join your grandfather" before pulling his arm out and leaving Zenitsu to die.

Nezuko, inoskue, Aoi, and kanao received word about Zenitsu's death and how he was killed by none other than Tanjiro. They swore that they would take Tanjiro down and stop his rein of terror.

Now in modern days

Tanjiro still remembered the time when he had killed them in their previous lives. But that was his human side that was almost gone, Tanjiro was desperately injecting himself with the cure for demons to become humans so he can keep his humanity, but it wasn't enough, each serum would only last him 10 minutes, but his drug was ready, he now just needed to find Oyakata and the demon slayer HQ. But with his demon side, he knew he might kill them, but it was a risk that he must take in order for the slayers to kill him. But he needs to find it fast since he is running low on the serum and with his demon side growing stronger, the serum slowly becomes ineffective. His demon side was the one that forced him to kill the hashiras. But his human side needed to wait for a very strong set of hashiras because he knew even with the drug that would weaken him to the point he is killable, the hashiras wouldn't be able to kill him because his demon side would kill them still. And luckily his prayers were answered, with the reincarnated hashiras and his reincarnated old friends, they can kill him with the drug me made.

To be continued

Hi! This is just to show what had happened to his old friends in the past, well you've only seen Zenitsu but you can assume what he did with his other friends.

Modern secret 1: Demon king Tanjiro is coming to an end
Modern secret 2: I felt so much pain when I killed Zenitsu here
Modern secret 3: was it sad enough or should I make it sadder? Because I'm going to try and make it sadder

Well... anyway SEE YA NEXT CHAPTER!