Abura-akago

A small flame comes drifting in through the window of a house at night. It moves without hurry, crosses the room, and settles beside the paper lamp — and as it settles it stops being a flame and becomes a baby.

The baby leans in and starts drinking the lamp oil. It drinks the way infants do anything, with total absorption, not looking up, not stopping. When the oil is gone it becomes a ball of fire again and floats back out the way it came, and the house is dark.

That's the abura-akago, the oil baby, and it is one of the eeriest small things in the whole catalog — partly because it never threatens anyone. It just comes in, feeds, and leaves.

The crime it's serving time for

It has a backstory, and the backstory is a sentence.

There was an oil merchant in Ōtsu who stole oil from a Jizō statue at a crossroads. That's it. That's the offense. He took the oil out of a roadside shrine's lamp — from Jizō, of all figures, the gentle one, the guardian of travelers and of dead children, the statue people dress in red bibs.

For that he came back as fire. And then he came back again as this: a baby that cannot stop drinking oil, forever, in other people's houses. He is not punished with pain. He is punished with appetite, and with being very small, and with having to keep doing the exact thing that damned him.

He is also not alone. Japanese folklore built a whole little criminal code around lamp oil, and the abura-akago is the youngest of four convicts. Steal oil from a shrine and you become the ubagabi, a fire with an old woman's face. Steal it from a temple and you become the sōgenbi, a burning monk's head. Steal pressed oil and you become the abura-sumashi, exiled to a mountain pass in Kumamoto. Steal it from Jizō and you become an infant. Four thefts, four sentences, one commodity.

Why oil was worth a curse

This all sounds wildly disproportionate until you remember what lamp oil was.

Before electricity, oil was light, and light was the length of your day. It was expensive, it was rationed, and it was the difference between a household that could work or read or sew after dark and one that went to bed at sunset. Stealing lamp oil wasn't petty theft. It was stealing time out of someone's life, and the folklore treated it accordingly.

There is also a completely mundane explanation sitting underneath, and it's a good one. Lamp oil in that era was often rendered from fish. Cats liked it. Cats got up on their hind legs at the lamp and licked at it, silhouetted against a paper screen, and people saw something small and hunched drinking the light.

Read that silhouette one way and you get the abura-akago. Read it the other way and you get the bakeneko, the monster cat. Same house, same lamp, same shadow — two entirely different monsters, decided by whether the shape looked feline or looked like a child.