When I first started writing this book, I ran into a snag of what exactly was an Onmyouji (pronounced on-myo-gee) 陰陽師. There’s a lot of beliefs and religion mixed into the concept, but basically, an Onmyouji is a practitioner of magic and divination.
During the Heian era, they were government officials meant to protect the Emperor and citizens from harm. Harm being curses and youkai 妖怪, creatures that people believed roamed the streets at night. One of the most famous practitioners was Abe no Seimei.