Shuumatsu no Harem - Fantasia

It's year 998, and the world has been corrupted by darkness. The next head of the small border kingdom of Nagara, Ark, has feelings for his cousin, Aurelia, but a political marriage to the Crown Prince has separated them. In order to get her back, Ark desires immense power, but a mysterious dark elf, Rati, appears in front of him. In exchange for a promise, she will give him "the power to rule the world."