Mutsukabezaka (六壁坂) is a creature named by Rohan Kishibe, and appears only in the one-shot Thus Spoke Kishibe Rohan - Episode 2: Mutsu-kabe Hill.
Mutsukabezaka is a malignant supernatural creature known as a yokai. The peak of happiness for this creature is when they possess an individual and then die in front of somebody else, making the witness suffer from guilt as they think they somehow killed the person. At that point, the yokai possesses the witness while making its previous host bleed forever. By exploiting people's love and the weakness of their heart, it makes humans do all the work when possessing them, so the yokai doesn't need to have any labor or responsibility itself.
According to Rohan Kishibe's research, Mutsukabezaka is a yokai from possibly thousands of years ago. Its only purpose is leaving descendants. The few Mutsukabezaka yokai left on Earth may be anywhere, dying in front of someone on purpose. Gunpei Kamafusa was possessed by one Mutsukabezaka, who made him die in front of his lover and impregnated her with its descendants.
After learning about Gunpei's story, Rohan investigates the region around Naoko Osato's house to discover his goal, suspecting him to be a yokai. Rohan comes across a young girl, who tells him she's not allowed to talk to strangers and runs away. Rohan tries to catch her, but she ends up falling on the ground and hitting her head on a rock, bleeding profusely like Gunpei did when he died. Rohan realizes this girl is actually Naoko and Gunpei's daughter, surprised that the man who became a corpse was able to impregnate her after his death. He discovers that the girl is possessed by a yokai, which tried to die in front of him in order to possess him. With Heaven's Door, he writes one last memory before the girl dies: "I don't know Rohan Kishibe. Even if I meet him, I won't see him". The yokai briefly shows its true form in order to possess Rohan, but stops its attack and returns back to the girl's body after being unable to see him. The girl remains alive due to the yokai's return and leaves the area, while Rohan comments to himself that if he was one second late, he would have been possessed or exterminated by the yokai. He names the yokai 'Mutsukabezaka' and declares his research complete.