Hengen Taima Yakou Karura Mau! Gaiden: Abe no Seimei-hen

It's the Heian Era in Ancient Japan. After numerous disasters and rebellions, The Emperor Suzaku believes Heaven has abandoned him and his advisors secretly consider ways to replace him. Meanwhile, the sorcerers known as Onmyouji are dragged into a series of fights against demons summoned by mysterious curses. They will have to delve deep into conspiracies to discover the true nature of these cases.

(Source: MangaDex)