ERIS NO SEIHAI S

Constance Grail, known as Connie, is a plain and ordinary Viscount's daughter, and Scarlet Castiel, the daughter of a duke who was once executed as an unparalleled villain. Connie is saved from a difficult situation by Scarlet's ghost, but in exchange, she must continue to accompany her on her quest for revenge. Due to certain circumstances, Connie and her friends end up visiting Elsor, the homeland of Scarlet's late mother. According to Scarlett, this is where "the Star Crown of the Faris Empire's last princess rests." (Source: Drecom, translated) — Description from AniList community (anilist.co).