Sō'unga

Sou'unga is a fictional sword in the anime and manga series "InuYasha". It is the complete opposite of Sesshomaru's inherited sword, Tenseiga, "the sword of life or heaven", and the more powerful form of Inuyasha's inherited sword Tetsusaiga, "the sword of death or earth".

Sou'unga, "the sword of hell or of world conquest", can summon 100 dead souls in one swing. In the third movie of InuYasha, the combined powers of the Tetsusaiga and the Tenseiga succeeded in defeating Sou'unga, which was stronger than either of them alone.

Sou'unga, unlike the Tessaiga or Tenseiga, was not created by Totosai from Inuyasha and Sesshomaru's father's fang and it is unknown how Inuyasha's father came to own it. Also, unlike the other two, Sou'unga contains the spirit of an ancient evil dragon from the depths of hell. When held by a human, Sou'unga takes control of them, and they will kill everything they possibly can, but when held by a yōukai, then Sou'unga will try to persuade them to do its bidding. But Inuyasha's father never let Sou'unga control him. Saying it was an act of weakness if he let what he should be controlling take control of him. He used the sword to fight Takemaru of Setsuna as a flaming mansion came down upon them.

Afterwards, Inu no taishu's servants Myoga, Totosai and Saiya pondered on what to do with Sou'unga, after disposing of the Tetsusaiga and Tenseiga. They couldn't give it to Inuyasha, as he was merely a baby at the time and neither Sesshomaru, who desired power. Saiya suggested he'd hold the sword off quietly for seven hundred years, placing it in the Bone Eater's Well. But skipping a number of years, the sword ended up in Kagome's family possession.

When Inuyasha grasped Sou'unga, he was taken over, but regained control for a short period of time. To prevent it from harming anyone in the future, Inuyasha went into the past, where it began a reign of blood. It battled Sesshomaru, knowing all of his thoughts when their swords clash. Kagome managed to release Sou'unga from Inuyasha's arm using the prayer beads, causing the sword to fly off.

Picked up by a lesser demon it had previously slain, Sou'unga went into the netherworld, retrieving Sesshomaru's left arm that was cut off by Inuyasha during their battle (Episode 7 of the series) and then came upon the burial sight of Takemaru, resurrecting him, giving him the arm so he could use the Dragon Twister and a chance for revenge on Inutaisho's sons. Grasping the sword, Takemaru gained a demonic appearance and slaughtered an entire castle full of soldiers to resurrect as the living dead. Sou'unga's spirit guided Takemaru, as Inuyasha and Sesshomaru's groups came upon the castle. Capturing Kagome and Rin, Takemaru tried to strike, but Sesshomaru's intervention stopped it. Sesshomaru tried to kill Takemaru with Tenseiga but he was resurrected. But when faced by Inuyasha, his desire to protect Kagome filled his power to push Takemaru back. Realizing that his love, Izayoi wanted him to live back then, leaves the living. But Sou'unga wasn't finished yet and constructed a body for itself to fight, opening a portal to the Netherworld that would encompass the living world. Sou'unga fought against Inuyasha and Sesshomaru, who battled it individually, only making it stronger. But they combined their Backlash Wave and Dragon Strike to destroy Sou'unga's body, sending it into the netherworld, where it can't harm anyone.

Special abilities

 * Dragon Twister: Sou'unga's most powerful attack, it posseses more destructive power than Tetsusaiga's Backlash Wave. Sou'unga unleashes a large, violent, and powerful purple/black tornado that annihilates everything caught in its path. Myoga described this as being the attack of a Dragon from Hell. Its strength is so great it can easily bowl-over the Backlash Wave. In the Third movie, Inuyasha declares he has someone to protect, thus giving him more strength so he can perform a stronger Backlash Wave. This Backlash Wave, also coupled the combined power of Sesshomaru's Dragon Strike, was able to finally destroy the spirit of Sou'unga.It is also called Gokuryuuha
 * Sheath of Sou'unga: Kept Sou'unga dormant for what seemed like seven hundered years. Saya inhabits the sheath, whom advised Myoga and Totosai to throw him with Sou'unga sheathed into The Bone-Eater's Well. Like the sheathes of Tetsusaiga and Tenseiga, it has powerful defensive capabilities.
 * Resurrection: When Sou'unga is in control of a "Lesser" yōukai, he can bring back a corpse from the dead, and control it, or give it free will. Mindless corpses are filled with deadly liquified miasma.

Trivia

 * Sou'unga can be considered cannon, because in the Final Act when we learn the truth about Tenseiga, it would thus mean Sou'unga could have never existed