The Baku, otherwise known as the ‘dream eater’, is a mythological being or spirit in and Japanese folklore which is said to devour nightmares. The Baku cannot be summoned without caution, however, as ancient legends say that if the Baku is not satisfied after consuming the nightmare, he may also devour one’s hopes and dreams.
Legend has it, that a person who wakes up from a bad dream can call out to Baku. A child having a nightmare in Japan will wake up and repeat three times “Baku-san, come eat my dream. Baku-san, come eat my dream. Baku-san, come eat my dream.” Legends say that the Baku will come into the child’s room and devour the bad dream, allowing the child to go back to sleep peacefully. However, calling to the Baku must be done sparingly, because if he remains hungry after eating one’s nightmare, he may also devour their hopes and desires as well, leaving them to live an empty life.
Cited from: https://www.ancient-origins.net/myths-legends-asia/baku-legend-dream-eater-002383