Superb Song of the Fate-Defying Valkyrie

A mysterious stranger comes to Castle Gyges in the kingdom of Valla, seeking aid in saving a world which is doomed to fall to an ancient evil. Expecting to gain the aid of a hero who made his own fate, is caught by surprise when the child of said hero volunteers in his place. Only one question remains: will she be able to carry the heavy burden she's willingly putting upon herself?