Neppu Kairiku Bushi Road

is a Japanese media franchise created by the late Japanese light novelist Sunao Yoshida

Plot
The world is in the pits of despair after being ravaged by a poison called "shinobi" that came to earth via an asteroid. People have run away to a place called "Kairiku" that used to be located at the bottom of the ocean. Here, the people try and protect themselves against the effects of the poison.

There is a single light of hope for people: the "Holy Weapon Giga Road" and the legendary warrior "Yagyuu." These two are needed in order to counter the effects of the poison and, in order to save the world, Ame, a princess of a fallen nation, stands up.

Official
Official Site

Other Sources

 * Wikipedia
 * AnimeNewsNetwork