Kagetoki-sama no Kurenawi Koukyuu

The manga's story centers on Tsutsuji, a high school girl nicknamed "Shinigami" because of her sinister personality. She steps to another world called the Pearl Country, where she confronts troubles and battles inside and outside the inner palace with a black prince named Kagetoki, who himself likes disturbances. Together, they form the strongest and worst couple.
(Source: Anime News Network)