Luce to Shiro no Keiyaku

Luce to Shiro no Keiyaku summary:

A brand new fantasy manga by Mikanagi Touya, the creator of "Karneval"!After losing her foster parents, Luce lives together with her younger sister, until suddenly one day, a woman named Athesia appears before her with an unusual request: "Would you be willing to become the lover of my fianc?" She is then abruptly taken to meet Rioverna, the young head of the Gailenche family - one of the nation's five great families. What follows is the establishment of a strange contract between the two?!