Youhen Nibelungen No Yubiwa

The story is set in a world were gods control the heavens, giants the earth, and nibelung (a strange looking race) the underground.
The main character (Sigurd) wanting to learn what fear is makes his entrance during a battle between the (almost extinct) humans and giants. Things weren’t looking good until he pulls out a broken sword that scares even the giants. At the same time, a goddess descends along with a meteor shower.
What is the importance of the broken sword who can only be fixed by “the fool without fear”? What’s the mission of the goddess on earth?