The Sage of the Six Paths (Rikudou Sennin) was the legendary figure who founded the arts of ninjutsu and created the ninja world. He defeated the Ten-Tails and sealed it within his own body, thus becoming the first jinchuuriki.
According to Jiraiya, the Sage was a monk who appeared long ago in an age of endless war. He was the first to discover the truth of chakra, and with it he tried to lead the world to an era of peace instead of conflict. He travelled across the land, spreading is ideal and religion, the Shinobi Sect, which eventually came to be known as ninjutsu. His great deeds and ultimate desire to bring peace to the war-torn world made him widely known as the Saviour of this World.
Centuries before the beginning of the main storyline, the sage defeated the most powerful demon in the ninja world, the Ten-Tails, and was praised as a god for the virtue of his victory. In order to protect the world from the Ten-Tails, the Sage developed a certain technique to seal the beast within his own body, giving him the ability to harness and control its overwhelming power; he became the first jinchuuriki. However, this offered only a temporary solution. In the event of his death, the seal would break and the Ten-Tails would one again be unleashed upon the world.