Tanjiro doesn't even know how this happened. He was fighting with a demon and got injured- he likely passed out, and he woke up in some mansion.

Tanjiro had felt different when he woke up. Hungry, undeniably so, but for what he didn't know. Yet when he looked at himself in a mirror-

He was a demon.

Tanjiro had been turned into a demon and he doesn't even know how. And he's not happy about it. Tears roll down his cheeks, gripping at his forearms.

And the door opens.