Long ago there was a monk of great virtue, the Zen master Kaian. After finishing the summer retreat at Ryutaiji in the province of Mino, he traveled toward the northeast and came to a village called Tonda in the province of Shimotsuke. Night was falling, and he tried to ask for lodging at a large house, but the moment the villagers saw him they shouted, "The mountain demon has come!" and ran to hide.
The master of the house explained. "On the mountain above this village there is a temple, where a learned abbot lived. But the abbot came to love dearly a boy of twelve or thirteen he had brought from the province of Koshi, and when the boy died of illness, in his grief he would not bury the body but held it skin to skin, and, sorry to see the flesh rot away, at last sucked the flesh and licked the bones clean. Since then the abbot has become a demon, coming down to the village night after night to dig up graves and eat the flesh of the dead."
"I will save that monk," the Zen master said, and set off for the mountain temple. The temple was in ruins, its buddha images toppled. When the master asked for a night's lodging, the gaunt abbot appeared, said, "Do as you like," and went inside.
Late at night the monk searched the temple for the master to devour him, but could not see him, though he was right before his eyes. At dawn the monk realized that the master had been sitting there all along, and, ashamed of his sins, prostrated himself.
The master placed the blue hood he was wearing on the monk's head and gave him a Zen koan of two lines: "The moon shines on the inlet, the wind blows in the pines. This long, clear night, what is it for?" "Sit here until you grasp the true meaning of these lines," he said, and left the temple.
A year later, when the master visited the temple again, in the ruined hall sat a monk as thin as a shadow, wearing the blue hood and faintly chanting the two lines. The master shouted, "What is it? What is it for?" and struck him on the head with his staff, and at once the monk's body vanished like ice melting in the morning sun, leaving only the blue hood and white bones. The monk's long attachment had at last dissolved. It is said that the master became abbot of the temple, and it flourished as a temple of the Soto school.