Usurper/Stolen Kings

The story of Kazue Nakanishi, the great-grandson of the great onmyouji Abe no Seimei, and how his life turned around after deciding to side with the very creatures he's been taught to control and exterminate.
The inhabitants of Kuromori's Nakanishi Castle are immortal and have long-lived through many events (that they were probably personally involved in, too, but shh.), but the Nakanishi's reign is threatening to come to an end once the fox, Daichi's, 900th birthday creeps up as the years go by.