Onmyouji Abe no Seimei

The novel series' story centers on the historical figure Abe no Seimei, one of Japan's most famous onmyouji (ancient Japanese occultist). The story of the first book begins when Abe no Seimei inadvertently assists Princess Tachibana during Kyoto's Kamosai festival, and finds out that she is possessed by a formidable spirit

(Source: ANN)