A Japanese magecraft force in possession of a Divine Body—a living fragment of a god.
The number of people directly contracted to the Divine Body is slightly short of 100, but that only makes them the third biggest magecraft force in Japan at best.
The name Yakou means “night eon.” In simpler terms, it Yakou signifies an endless night. And as this name suggests, their god’s nature is intimate with the Japanese underworld.
To pay respect to their god, all members of the organization generally wear black, be it on a suit or a kimono.
They had deep ties to gambling dens, and because there's little to no need to protect people from fiends in the modern day, managing the gambling houses now became their main activity.
Although limited to a few locations, their Boundaries are extremely vast and they have powerful techniques that can be used only within their Bounded Field, thus giving them unparalleled defensive power.
Incidentally, Japanese organizations never say the name of their gods without reason or use an alternative name known only within the organization. This is an attempt to prevent the deterioration of arcanity by not banalizing the name. The Four Demon Hunter Families of old were believed to have employed this same method.

TYPE-MOON BOOKS material: The Adventures of Lord El-Melloi II Glossary