Kuzushi no Oni

In 1123 when a solar eclipse released the Oni no Kuzushi from it's prison in the spirit realm after two hundred years of imprisonment it found itself bound to a forgotten estate deep within the Phoenix woodlands. The oni used it's powers to unleash a leathal plague spread with magic, known as the Darkfever Plague, and many lives are lost until a group of clever Magistrates locate and destroy the oni.

(3E p. 17)