About the Sword

Sou'unga, unlike Tessaiga and Tenseiga, was not forged by Toutou-sai. It is a sword that possesses an ancient, evil spirit and when a human has it Sou'unga will make them destroy everything. Since he was the only one powerful enough to control it and so many humans were trying to rule the world with it, it was entrusted to Inutaishou.

While Tenseiga is the sword of heaven and Tessaiga is the sword of man, Sou'unga is the sword of hell. Tenseiga can save a hundred in one swing, Tessaiga can kill a hundred in one swing, and Sou'unga has the power to open hell and call forth a hundred of the dead with one swing. Sou'unga can beat Tenseiga and Tessaiga independently, but if their powers are combined Sou'unga gets beat.

While Tessaiga has Bakuryuuha and Tenseiga (apparently) has Souryuuha, Sou'unga's ultimate attack is Gokuryuuha. I haven't seen the movie yet so I'm not sure if it's the same attack, but floating dragon heads come off of the sword and attack the opponent.

Saya and the Sheath

After Inutaishou's death, the sword was sent down the well and put into the care of the Higurashi shrine. Saya, some sort of spirit that guards the sword, was only able to keep Sou'unga sealed for two hundred years. He tells the family the 700-year-seal is about to be broken, which is two hundred years at the shrine plus the five-hundred-year jump into the past.

Sou'unga's scabbard can be seen as the good force keeping Sou'unga's evil powers at bay. It houses Saya and can be used to form a protective barrier.

Role in Tenka Hadou no Ken

After blasting his way into the castle and reviving Izayoi, Inutaishou uses Sou'unga to fight with Takemaru and dies. Myouga, Toutou-sai, and Saya get together to decide what to do with his swords, and toss Souï¿1⁄2unga down the well.

Seven hundred years in the future, Grandpa Higurashi brings Sou'unga out from storage and mistakenly calls it 'Murakumo no Tsurugi'. Souta goes to touch it, and Saya pops out to warn them about Sou'unga's seal. The sword does flying off and frees itself from the sheath, imediantly going to work on destroying the city. Inuyasha grabs hold of it to stop it, and purple tendon-things dig into his hand and wrap around his arm. Sou'unga possesses him and he turns to his red-eyed demon state, but he fights it off long enough so that he doesn't hurt anybody.

Sou'unga is throw off of Inuyasha by the force of the sitting necklace, and an oni picks it up. The sword wants a better user so it possesses the oni, enters the youkai graveyard, and takes Sesshoumaru's left arm. It visits Takemaru's grave and revives then possesses him, changing him into a sort of half-youkai thing. They enter a castle and destroy the returning army, turning them into zombies and sending them after the Inu-tachi and Sesshoumaru-gumi

When Takemaru dies (again), Sou'unga uses Sesshoumaru's arm to make itself its own body and continues to fight. It is finally blasted by both Inuyasha and Sesshoumaru simultaneously and goes flying. Inutaishou makes his entrance and tells them Sou'unga has been sealed in the underworld forever before he disappears again.