Ninja were soldiers and mercenaries in feudal Japan who were typically hired by daimyos to carry out stealth missions. Japanese folklore states that the first ninja descended from a yokai that was half-man and half-crow. Yokai are supernatural monsters or demons that are known to shapeshift and terrorize the living. The practice of wearing yokai masks, like oni masks, has been a Japanese tradition since the Jomon period. Today, oni masks are commonly worn during festivals, where they are used to frighten children and ward off evil spirits.