Gamaran

Gamaran (我間乱, Gamaran) is a manga written and illustrated by Nakamura Yousuke. It was serialized in Weekly Shonen Magazine from May 2009 to June 2013.

Plot
"Middle Edo Period -- One state becomes the gathering place for sinful martial artists, who believe in nothing other than power, know no other way of life than letting oneself go wild in battle. That state is the Unabara State, the "Haunt of Demons"..."

Washitzu Naosata, the ruler of the powerful state, Unabara, is looking for his successor. He calls upon all 31 of his sons and tells them to search for whoever they believe to be the strongest martial artist of all. Each of their chosen martial artists are then to fight against each other - until only one remains. Whoever is the winner, whoever has the strongest martial artist in the nation by his side, will become his successor...

Out in the midst of the mountains is the doujo of "Daigame Ryuu" (Giant Tortoise Style). There lives Kurogane Gama, the son of the legendary swordsman, Kurogane Jinsuke, who is believed to have killed 1000 martial artists in Unabara. There, one of the sons of Washitzu Naosata, Washitzu Naoshi, arrives in search of the legendary swordsman, only to find that he has been missing for several years. After seeing the battle style of Gama, Naoshi decides to ask Gama to come with him instead. Wanting to become stronger, Gama agrees with his offer, and that marks the beginning of Gama's battle to become the strongest of all...