Onmoraki is a monster bird written in Chinese and Japanese classics. According to a Buddhist scripture, "Kyoten (Buddhist Scripture) Daizo-kyo Sutra" (the Tripitaka), it was the transformation of a spirit that rose from a new corpse. It was the transformation of a corpse that did not receive a proper kuyo (a memorial service for the dead) and said to have appeared before a monk that failed in reading Buddhist manuscripts.
Its was depicted in a diagram within classics such as the art book by Sekien TORIYAMA, "Konjaku Gazu Zoku Hyakki" (Continued Illustrations of the Many Demons Past and Present), in a note quoted from a Chinese classic "Seisonroku," and its appearance was like that of a crane, having black body color, and its glittering eyes were like flame, and it sang in a high pitch note while shaking its wings.
The following story of Chinese onmoraki is included in the "Seisonroku." It was during the period of Sung Dynasty. A person named "崔嗣復" (Shifuku SAI) of Zhengzhou city woke from a voice that shouted at him while he slept on top of a hodo (treasure hall) of a temple outside the capital city. There was a monster bird whose appearance was similar to the one described before, and it disappeared when Sai escaped. When Sai asked a monk of the temple about what had happened, there was no monster there as such but they temporarily stored a body of the dead several days ago. Upon returning to the capital, he asked a monk of a temple about the bird, who then told him that it should be an onmoraki that was born transmuting from the spirit of the newly dead.
A story of Onmoraki appears in "Taihei hyaku Monogatari" (100 Ghost Stories), written during the Edo period, which is similar to the one in the "Seisonroku." When a man named Takube drifted off to sleep at a temple in Yamashiro Province (present-day Kyoto Prefecture), he was awaken by a voice calling out his name. He then saw a monster bird there. Surprised Takube escaped, and while he saw the bird from his hiding place, it disappeared. When Takube asked the elder of the temple, he was told that "Kyoten Daizo-kyo Sutra" stated that the spirit of the newly dead would transform into Onmoraki, and it might be caused by the corpse temporarily stored at the temple recently.
The name Onmoraki was reportedly created by adding words 'in' (negative) and 'oni' (ogre or ghost) to a monster 'mara' that hindered Buddhists from attaining enlightenment, to put stress on the meanings of ogre and monster, while another theory has it that it was a mixture of 'Onmo' meaning obstacles and 'Rasetsuten' (Rakshasa, an evil spirit).