The Japanese folklore tradition of a night procession featuring countless yōkai is called Hyakki Yagyō.
Hyakki Yagyō is commonly translated as “Night Parade of One Hundred Demons.” In the tradition, supernatural beings emerge and travel through the night. Anyone who encounters the procession may face danger, although later stories and artistic works vary widely in how literally or dramatically they treat the event.
The idea became especially influential through medieval and early-modern Japanese picture scrolls and illustrated books. Artists used the parade to display collections of strange beings, including animated household objects and more monstrous creatures. The phrase therefore describes both a supernatural event in folklore and a major theme in Japanese yōkai art.
The number one hundred is not necessarily a literal headcount. In Japanese expressions, hyakki can suggest a multitude of demons or strange beings. Hyakki Yagyō should also be distinguished from Obon, a Buddhist observance for ancestral spirits, and from an ordinary matsuri procession.