Ikigimo (生き肝, lit. "the liver taken from a still-living animal") is a term for the act of eating a human's liver in order to gain power. Supposedly it came about based on an old story from China, where yōkai sought out the ikigimo of Buddhist Sanzō priests. It was a common belief among yōkai of Kyōto 400 years ago. Yōkai believed that the more sacred life forces, such as those of infants, priestesses, and imperial princesses, would increase their strength. One yōkai claims that eating an infant's liver provides the strength of 100 spirits.
Hagoromo Gitsune sought out princesses with special abilities for just this purpose, 400 years ago. Supposedly, eating the heart of a still-living animal serves a similar function.