Shironeko Project: Zero Chronicle

A long time ago, there is an ancient legend revolving around two figures, the White Maiden of the Sky; represented by the White Cat, and the Black Prince of Darkness; represented by the Black Cat. At the same time, a war between the demons and the guardians of light took place, with the demons led by the Black King of Darkness attacking the lands in the sky. The Black Prince however, despite the wishes of the Black King, defied his orders and falls in love with the White Maiden, in hopes that their union would end the war. Both were not meant to meet and love each other as the war ravaged the world around the land and the skies.

(Source: Wikipedia)