The Tenth Pillar. (Kimetsu No Yaiba X OC)

Heroes always arrive late.

Hiiro Kojima stepped out of the Senkaimon. The Butterfly estate was as silent as ever, as beautiful as every day. At least, he was back.

There were no Reiatsu from the idiots nor Shinobu's inside the estate. The only ones there were the little girls and Aoi.

"Hiiro-sama?" The blue butterfly girl was surprised about the fact that the hashira was still there. "Why are you here? All the pillars were summoned by Oyakata-sama a while ago."

The tenth pillar sighed, he was late. He cursed his luck as he concentrated his Reiatsu. He needs to find them as soon as possible. There, he found them in another place... like another dimension.

"Aoi, bring me some medical supplies. We may need them." The young nurse ran outside the estate. That was a good opportunity to get his door to the location of the other pillars. "If we are talking about a different dimension..."

He closed his eyes, he wanted to see him and for that he'll need to travel to the deepest part of his soul.

'What do you want?' The white haired Hiiro asked annoyed, he was currently sleeping and did not appreciate the disturbance of his sleep.

The night pillar explained the situation.

'Ha? Just that? Idiot, just open it and you can go. I don't care at all if you use a little of my powers.'

Hiiro Kojima smirked, he was his natural enemy but during the hellish training, he became more of a nakama. He will need him later after all.

"Here are the supplies!" The blue eyed girl handed out a small bag, it was light but he knew that the girl would put in everything he may need. He patted her head and turned around.

"Thanks. I'll be going now." He unsheathed his black blade and made a horizontal cut into the air. "Garganta."

The space distorted and twisted, soon a black and gray portal began to open slowly. It was a pitch black space, there was not even a dim light inside. The night pillar stepped into the garganta.

He warmly smiled at the girl, he felt her doubts on her heart, it was a fear and even a little despair.

"I'll bring them back. I promise." The portal slowly closed as the night pillar began his journey to the Dimensional fortress.

When he arrived to the labyrinthine fortification, he found himself at the center of it. He felt multiple Reiatsus inside. He felt the water pillar and Tanjiro's, they were fighting the Upper Moon three, he also sensed that Sanemi was fighting against Upper Moon one, that was his target.

"Well, let's rock!" Before he could take a single step forward to his battle against Kokushibo, Shinobu's Reiatsu began to dissapear, slowly. "Wha—"

Hiiro didn't even thought about it, he used the shunpo to reach the room. He could navigate through the fortress easily, he just needs to follow the Reiatsu of her beloved Shinobu.

As he ran, he saw the battle of Zenitsu against a black demon  the blonde boy unleashed a never seen technique: The seventh style of the lightning breathing, with a fast swing, his young disciple sliced the demon's neck, killing him instantly.

The raven haired boy felt proud of the young boy, he has really grown stronger.

As he entered the room, his eyes refused to believe the scene in front of him: Shinobu was being absorbed by the Upper Moon two.

Kanao rushed trying to save her sister but it was no use, she wasn't fast enough, but Hiiro was, he was faster and also he was angry, no, he was furious.

"WHAT ARE YOU DOING?" He barked as he launched herself, unleashing the first style of his breathing: the night slash.

Douma felt his instincts alerting him, a black blade was coming directly to his neck, he won't be able to dodge if he continued to absorb the insect pillar.