Koukyuu Tenkou Monogatari

Princess Hinaka, who has a faint crush on the military general Shikou, aspires to become a "Heavenly Empress" who can be granted special divine powers in order to help her childhood friend. However, one day, Shikou suddenly kills Hinaka's brother, the current emperor Kuroren, and usurps the throne. He then tells Hinaka, "From today, you are my wife," and holds her captive in the harem. In order to uncover the truth behind the sudden change in the person she loved, Hinaka struggles to "escape" from the harem, but...!?

(Source: Akita Shoten, translated)