A great many years ago, when the Earth was still young...
... The world served as a mighty battleground. A war between 12 combatants. These warriors were incredible beings, having unmatched strength and power. Of these warriors, one of them was the Great Dragon God, Ryujin. Ryujin was a powerful warrior, but also possessed a pure heart. From his scales, he formed humans in his image. These humans were gifted, blessed with the strength of their father, Ryujin. They took after his fluid movements and amazing capabilities, and formed a clan of master fighters.
They formed the Dragon Ninja Clan.
They were tasked by the almighty Ryujin and his dragon brethren to combat and seal away the evil Tsuchigumo, the Spider God, and his evil followers, the vile Fiends in their own world, the Fiend Realm. For this task they were given his left fang. They forged this fang into an almighty sword, the Dragon Sword. Not only was it able to masterfully cut through fiends, but it also served as a key, used to lock the gates to the Fiend Realm. Once this task was done the Fiends that had remained on Earth needed to be sealed away as well, but they couldn't be sealed into the Fiend Realm, as it was already closed.
So for this task, Ryujin gave the Dragon Ninja Clan his right fang. This was forged into a new sword, the Dark Dragon Blade, which could be used to absorb the souls of evil Fiends. This sword could not be controlled, so the Dragon Ninja Clan was tasked with guarding this weapon, for if it got into the hands of evil there would be no hope for mankind.
Now, the Dragon Clan is one of the clans in the Jyuni Togatta Suta, a system of keeping tabs on the Twelve Ninja Clans. To this day they guard the Dark Dragon Sword, using their might of the Dragon to slay fiends and enemy ninja.
