Sō'unga

is a demon sword. It is the opposite of Sesshōmaru's inherited sword, Tenseiga, "the sword of life or heaven," as Sō'unga is considered "the sword of death or hell."

Sō’unga, "the sword of hell or of world conquest," can summon 100 dead souls in one swing. The combined powers of the Tessaiga and the Tenseiga are the only swords capable of defeating Sō’unga, which was stronger than either of the two swords alone.

History
Sō’unga, unlike the Tessaiga or Tenseiga, was not created by Tōtōsai from the Inu no Taishō's fang and it is unknown how Inuyasha's father came to possess it. Also, unlike the other two, Sō’unga contains the spirit of an ancient evil dragon from the depths of hell. When held by a human, Sō’unga takes control of them, and they will kill everything they possibly can, and when held by a yōkai, Sō’unga will try to persuade them to do its bidding. However, if the yōkai is strong enough, they can overpower Sō’unga with their own will and control the sword on their own. One such demon was Inuyasha's father, who said 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 collapsed and killed them both.

Afterwards, the Inu no Taishō's servants Myōga, Tōtōsai and Saya pondered on what to do with Sō’unga, after disposing of the Tessaiga and Tenseiga. They couldn't give it to Inuyasha, as he was merely a baby at the time and neither Sesshōmaru, who desired power and would kill them for being given Tenseiga. Saya suggested he'd hold the sword off quietly for seven hundred years, placing it in the Bone Eater's Well.

During the story
However, about 200 years later, the sword ended up in Kagome's family possession. In the 21st century, the seal became broken. Therefore, Saya tried finding Inuyasha, in hopes that he would be able to control Sō’unga before it could unleashed chaos in the modern world.

When Inuyasha grasped Sō’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 Sesshōmaru, knowing all of his thoughts when their swords clash. Kagome managed to release Sō’unga from Inuyasha's arm using the prayer beads, causing the sword to fly off.

Picked up by an undead ogre it had previously slain, Sō’unga went into the netherworld, retrieving Sesshōmaru's left arm that was cut off by Inuyasha during their battle (Episode 7 of the series) and then came upon the burial site of Takemaru, resurrecting him, giving him the arm so he could use the Dragon Twister and a chance for revenge on the Inu no Taishō's sons. Grasping the sword, Takemaru gained a demonic appearance and slaughtered an entire castle full of soldiers to resurrect as the living dead. Sō’unga's spirit guided Takemaru, as Inuyasha and Sesshōmaru's groups came upon the castle. Capturing Kagome and Rin, Takemaru tried to strike, but Sesshōmaru's intervention stopped it. Sesshōmaru tried to kill Takemaru with Tenseiga but he was resurrected. However, when faced by Inuyasha, whose desire to protect Kagome filled him with power, Takemaru was back. Realizing that his love, Izayoi wanted him to live back then, Takemaru finds peace and leaves the world of the living. Sō’unga, however, wasn't finished yet and constructed a body for itself to fight, opening a portal to the Netherworld that would encompass the living world. Sō’unga fought against Inuyasha and Sesshōmaru, who battled it individually, weakening it with Tenseiga and Tessaiga's combined presence but unable to defeat it. However, they eventually combined their Backlash Wave and Dragon Strike techniques to destroy Sō’unga's body and eradicate the demonic spirit, sending it into the netherworld, where it couldn't harm anyone.

Special abilities

 * Dragon Twister (獄龍破, Gokuryūha; lit. Hell/Prison Dragon Blast): Sō’unga's most powerful attack, and it possesses more destructive power than Tessaiga's Backlash Wave. Sō’unga unleashes a large, violent, and powerful purple/black tornado that annihilates everything caught in its path. Myōga described this as being the attack of a dragon from Hell. Its strength is so great that it can easily bowl-over the Backlash Wave.


 * However, if both Tessaiga and Tenseiga are close by, the strength of the Dragon Twister is diminished. In the Third movie, Inuyasha declares he has someone to protect, this determination thus giving him more strength so he can perform a stronger Backlash Wave. This Backlash Wave, also coupled the combined power of Sesshōmaru's Dragon Strike, was able to finally destroy the spirit of Sō’unga.


 * Sheath of Sō’unga: This kept Sō’unga dormant for what seemed like seven hundred years but was actually only 200 due to the Bone Eater's Well transporting it forward 500. Saya inhabits the sheath, whom advised Myōga and Tōtōsai to throw him with Sō’unga sheathed into The Bone-Eater's Well. Like the sheathes of Tessaiga and Tenseiga, it has powerful defensive capabilities, though it is unknown if it was made from Bokusenō.


 * Resurrection: When Sō’unga is in control of a "lesser" yōkai, he can bring back a corpse from the dead, and control it, or give it free will, even able to counteract the powers of Tenseiga. Mindless corpses are filled with deadly liquefied miasma.


 * Black Spikes: Sō’unga can fire a shower of black spikes to impale it's enemies much like Inuyasha's adamant barrage.


 * Path to Hell: Sō’unga can open a portal in the underworld that in time it is able to absorb the souls of the living world and also absorb the whole world into oblivion.


 * Self-sustaining body: Out of desperation, Sō'unga can mold a physical body of armor, though it lacks a head. While it possesses no proper arm, its left one is a giant curved blade and wields it effortlessly with precision and speed.

Trivia

 * It is voiced by Tachiki Fumihiko.
 * Sō'unga, along with Mistress Centipede, are the only other character aside from Kagome and Inuyasha to travel from the present to the Feudal era as well as the only antagonist.
 * Sō'unga, though while controlling its owner's and having a will of his own, has had more owners than any other sword or weapon in the entire series, (though not if you include the sacred jewel shards.)
 * Sō'unga bears many similarities to the fictional sword Soul Edge from the SoulCalibur series by Namco. Both are very powerful and sentient swords once wielded by heroic leaders (Inu no Taishō and Algol, respectively), possessed a malevolent spirit (Sō'unga being the sword itself and Soul Edge's being Inferno) and have the ability to brainwash and manipulate their hosts and both craft armor for them while doing so. In times of desperation, both can also take control by either using pieces of or whole beings of the dead as a potential host and became stronger and faster as a result of it and even have the same laterality, being left handed with their swords. Lastly, both have also possessed the main protagonist of the series and mutated their arms to a certain degree. They have also used similar weapons and fighting styles.
 * Sō'unga has the physical attributes of a chokuto (an ancient Japanese sword) most notably a shitogi tsuba and a large spherical pommel extending from the grip. This design is accurate with how old the sword itself is.