Tenshou shita Dai Seijo wa, Seijo de Aru Koto wo Hita Kakusu ZERO

It's three hundred years before Fia and her friends' adventures in Náv, and Princess Serafina, the blind second princess of the land, lives hidden away in the forest. With only the forest spirits and her cousin, the young knight captain Sirius Ulysses, to keep her company, Serafina leads a peaceful life. But one day, an incident causes mysterious powers to awaken within her, and Serafina takes her first steps on the path that will ultimately lead her to becoming the one and only Great Saint!