Ikki Yako

Ikki Yakō (一鬼夜行) is a oneshot manga by Eiichiro Oda written in 1993.

Plot
The story starts with a monk named Gukou walking through a forest looking for a place to spend the night. He then hears someone call him and is seemingly attacked by a monster. Gukou promptly runs away and makes his way out of the forest. He sees a village and goes to ask if he can spend the night there. He realizes right when he's asking that they may ask for his help in exterminating that monster he saw earlier, so he just asks if there is a temple nearby that he can do his ascetic practices at. His hunch is correct however, and to his dismay, the entire village asks for his help in slaying the monster cannibal, to which he begrudgingly agrees. The mayor of the village offers to let him stay at his house in the mean time, but the people from the first house he went to say that he needs to stay at the temple for his practices, and thus he is forced to stay there.

Once at the temple he is again seemingly attacked by the monster. He dodges the attack and begs for his life, telling him that he doesn't taste good. The monster asks him to move so that he can enter his home, the temple, and takes off the monster mask revealing that he is the Guardian Deity of the temple that has been there for three hundred years. He reveals that he was bored recently, so he was scaring humans with that mask to entertain himself.

They talk inside and Gukou reveals that he is looking for his master who had disappeared five years ago. Gukou expresses his relief that there is no monster and asks if there is any food, since he dropped his when the guardian had scared him earlier that evening. The guardian expresses confusion to this statement, saying he doesn't remember scaring him earlier. He then admits that he was scaring people in the west forest at that time, to which Gukou reveals that he was in the east forest. The guardian then says that another monk that was a friend of his, had come before to slay a cannibal before the guardian had started his scarings, and that there is probably a real cannibal. The guardian says that, as a god, he can't slay the monster to help humans. Gukou then decides to flee instead of fight, but is ambushed by the Cannibal. After being badly beaten by the monster, Gukou tries to reason with him. The cannibal explains that he hates monks, because earlier, another monk talked to him and then attacked him when he was off guard. In retalliation he ate that monk.

The guardian, who had come to the fight scene asked what the monk's name was, as he recognizes the clothes as the clothes of his monk friend. It is revealed that the monk's name was Koushin, and that the cannibal had taken the clothes after eating him. The guardian attacks the cannibal in a rage, but is stopped by Gukou who says that he shouldn't kill others in the human world. He then reveals that Koushin is the name of his master that he was looking for. Gukou take the guardian's sword, and after a brief fight, cuts the cannibal in half, killing him. From then on, Gukou traveled around exterminating monsters with that sword. The people who he saved described him as having a scar on his forhead, and having a sword, even though he's a monk.