Kimetsu no Yaiba Season 1 Episode 6.
Placing Nezuko in a box he carries on his back, Tanjiro travels with her to the town where young girls are vanishing at night. There he meets Kazumi, whose fiancee Satoko vanished the previous night. Kazumi takes him to where his girlfriend was last seen and Tanjiro begins to track the demon’s scent. Kazumi recalls how he was beaten by Satoko’s parents, who did not believe Satoko had simply vanished and assumed he had done something to her. He realizes Tanjiro is a member of the Demon Slayer Corps and that’s why he believed him. Night falls as Tanjiro continues to track the demon. A young girl is abducted from her bed by a demon who can take a liquid form to pass through floors, which Tanjiro notices. He can smell the demon and girl but can’t see them, realizing they are traveling underground. He stabs into the ground and wounds the demon, letting him retrieve the girl. The demon comes aboveground. It has morphed and become capable of “Blood Demon Art” spells to move through solid ground. The demon breaks into three beings, and between that and their high rate of regeneration, Tanjiro finds himself overwhelmed. The wounded demons taunt the humans, telling Kazumi they already devoured Satoko. Nezuko comes out of her box to help, evening the odds.
Urokodaki tells Tanjiro there is only one demon still alive who has the special blood needed to turn a human into a demon, and thus was the one who attacked the Kamado household. He is also the only one who may know how to turn Nezuko back into a human. He is over a thousand years old and the progenitor of all demons: Kibutsuji Muzan.