Sō'unga

was a demon sword. It was the opposite of Sesshōmaru's inherited sword, the Tenseiga, "The Sword of Life or Heaven," as Sō'unga was considered "The Sword of Death or Hell."

Sō’unga, "The Sword of Hell or of World Conquest," could summon 100 dead souls in one swing. The combined powers of the Tessaiga and the Tenseiga were the only swords that were capable of defeating Sō’unga, which was stronger than either of the two swords alone.

History
Sō’unga, unlike the Tessaiga or the Tenseiga, was not created by Tōtōsai from Tōga's fang and it was unknown how he came to possess it. Though Myōga hinted that Tōga obtained it to quell the conflicts that humans and demons made in order to have So'unga. Tōtōsai also said that the Great Dog Demon had it long before the Tessaiga or the Tenseiga were forged. Also, unlike the other two, Sō’unga contained the spirit of an ancient evil dragon from the depths of Hell. When held by a human, Sō’unga took control of them, and they would kill everything they possibly could, and when held by a yōkai, Sō’unga would try to persuade them to do its bidding. However, if the yōkai was strong enough, they could overpower Sō’unga with their own will and control the sword on their own. One such demon was Tōga, who said that 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 the flaming mansion collapsed and killed them both.

Afterwards, the servants Myōga, Tōtōsai, and Saya pondered on what to do with Sō’unga, after placing the Tessaiga in Inu no Taishō's tomb and the giving the Tenseiga to Sesshōmaru. They couldn't give it to Inuyasha, as he was merely a baby at the time, and they feared Sesshōmaru would kill them on sight for being given Tenseiga instead of the Tessaiga. Saya suggested he'd hold the sword off quietly for seven hundred years, and placed it in the Bone Eater's Well.

During the story
However, about 200 years later, the sword ended up in Higurashi family's possession. In the 20th century, the seal broke. Therefore, Saya tried finding Tōga's son Inuyasha, in hopes that he would be able to control Sō’unga before it could unleash 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, and knew all of his thoughts when their swords clashed. Kagome managed to release Sō’unga from Inuyasha's arm by using the prayer beads, which caused the sword to fly off.

After it was picked up by an Undead ogre that it had previously slain, Sō’unga went into the Netherworld, retrieved Sesshōmaru's left arm that was cut off by Inuyasha during their battle, then came upon the burial site of Takemaru, resurrected him, and gave him the arm so he could use the Dragon Twister and a chance for revenge on Tōga's sons. After he grasped 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 found peace and left the world of the living. Sō’unga, however, wasn't finished yet and constructed a body for itself to fight, and opened a portal to the Netherworld that would encompass the living world. Sō’unga fought against both Inuyasha and Sesshōmaru, who battled it individually, and weakened it with Tenseiga and Tessaiga's combined presence, but were 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, and sent it into the Netherworld, where it couldn't harm anyone.

Powers & Abilities

 * Sō’unga's most powerful attack; it possessed more destructive power than the Tessaiga's Backlash Wave. Sō’unga unleashed a large, violent, and powerful purple/black tornado that annihilated everything that was caught in its path. Myōga described this as being the attack of a dragon from Hell. Its strength was so great that it could easily bowl-over the Backlash Wave. However, if both Tessaiga and Tenseiga were close by, the strength of the Dragon Twister was diminished. In the Third Movie, Inuyasha declared that he had someone to protect; this determination thus gave him more strength so he could perform a stronger Backlash Wave. This Backlash Wave, also coupled the combined power of Sesshōmaru's Tenseiga, was able to finally destroy the spirit of Sō’unga.
 * Reanimation: Unlike other weapons which includes demon swords as well, Sō'unga's slain victims didn't simply die but their corpses are reanimated as soulless undead that unless specially commanded will mindlessly attack the living in their vicinity. When Sō’unga was in control of a "lesser" yōkai, he could bring the corpses of the dead or the recently deceased back to life and take control of them, or even semi-revitalize ones appearance and give it free will. When it does this, either he plants himself into a grave and call upon their spirit to inhabit their former bodies or have its blade release a blood red-like liquid that animates them. A power that's even able to counteract the anti-undead aspects belonged to those of the Tenseiga. Aforementioned mindless corpses were filled with deadly, liquefied miasma which, when at Sō’unga's full power, enable said specters to restore themselves from most kinds of damage.
 * Possession: It was said that if a human, weak demon, or half-demon were to grab hold of Sō'unga, then they would be under its control, and if a strong demon like a Daiyokai were to take the Sō'unga, then it would attempt to sway them to the best of its abilities to do its bidding. This was shown when Inuyasha was controlled by the Sō'unga when he attempted to place it back in the sheath by growing purple tentacles out of the hilt to wrap around the target's arm that takes hold of it. However it's proven that if the possessed individual has enough mental strength and willpower then they can momentarily resist Sō'unga's control as demonstrated by Inuyasha.
 * Illusion: The demon sword has the ability to greatly alter other's perception of reality at a staggering rate as just after unsheathing and planting itself onto the ground, it cause Inuyasha, Kagome, Saya and all the other humans in the city to perceive the surrounding structures crumbling into ruins but the city was restored back to normal once Inuyasha took hold of the sword.
 * Mind Reading: It has the ability to invade another's mind and obtain any useful information about them just by coming into contact with something his target is holding such as Tōkijin while Sesshōmaru was using it against Inuyasha and Sō'unga.
 * Phantom Castle: Similar to Naraku, the Sō’unga could utilize it's corrosive demonic power to not only reanimate the dead but also corrupt and alienate the environment around it. Creating a demonic, phantasm fortress from a once-human settlement that belonged to a general and his armies.
 * Dimensional Travel: Sō'unga could travel to the border between the living world and the Netherworld where the Tōga's corpse resided in order to retrieve Sesshōmaru's severed left arm to unleash the Dragon Twister. Though it seemed that it needed to possess a body to do this, as Sō'unga didn't make any attempts to travel there until it possessed the Undead ogre however it could also be attributed to the fact that it probably didn't have either the power or means create its self-sustained body then to retrieve Sesshōmaru's left arm itself.
 * Path to Hell: Sō’unga could open a portal in the Underworld that, in time, was able to absorb the souls of the living world and also absorb the whole world into oblivion.
 * Self-Sustained Body: Out of desperation, Sō'unga could mold a physical body of armor, though it lacked a head. While it possessed no proper arm, its left one was a giant curved blade and wielded it effortlessly with precision and speed. This could also be because it still possessed Sesshōmaru's severed arm.
 * Master Swordsman: While using a body that it had constructed on its own, Sō'unga was shown to be extremely skilled and masterful in swordsmanship where it effortlessly fought and overwhelmed both Inuyasha and Sesshōmaru at the same time while having only one arm (though their uncoordinated and uncooperative attacks made that very easy).
 * Enhanced Strength: Sō'unga in this form had enough strength to effortlessly push back Inuyasha and easily fended off Sesshōmaru's attacks.
 * Immense Speed: While in the form, Sō'unga was shown to be very fast where it could easily dodge and parry attacks coming from both Inuyasha and Sesshōmaru at the same time with little effort.
 * Immense Durability: So'unga was extremely durable where it was said by Saya that only the combined power of Tessaiga and Tenseiga can destroy it. This is proven when individual attacks from both Sesshōmaru and Inuyasha were unable to even scratch Sō'unga and only when they combined their Backlash Wave and Dragon Strike was the demon sword finally dealt significant damage and defeated.
 * Black Spikes: Sō'unga could fire a shower of black spikes to impale its enemies much like Inuyasha's adamant barrage though he seems to require continuously swinging his arm in the direction to fire the spikes while Inuyasha's adamant barrage only needed one slash.
 * Immense Demonic Power: Sō'unga is not only a powerful weapon, but also a powerful fighter with its own body with Saya stating that he's more powerful than either the Tessaiga or Tenseiga alone. It was able to overwhelm both Inuyasha and Sesshōmaru, who both surpass their father in power and narrowly destroyed the world itself by unleashing hell on Earth. It took both of them to unleash their most powerful attacks in order to defeat Sō'unga, arguably making it one of the most powerful foes they've faced. When its demonic powers are exerted, its aura takes the form of purple serpentine dragons swirling around the blade.
 * Corrosive Aura: While in his self-sustained form, Sō'unga's sword arm blade could emit a powerful demonic aura that could corrode even other powerful demon swords like Tōkijin that rendered it unusable for the time and also caused a small explosion on the object.
 * 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 years. Saya inhabited the sheath, and whom advised Myōga and Tōtōsai to throw him with Sō’unga sheathed into the Bone-Eater's Well. Like the sheathes of both Tessaiga and Tenseiga, it had powerful defensive capabilities, though it was unknown if it was made from Bokusenō.

Trivia

 * It was voiced by in Japanese, and by  in English. Both his Japanese and English voice actors also voiced the Tsukumo no Gama and Gamajirō. Sō'unga's seiyū also voiced The Paper Doll Salesman from Rumiko Takahashi's previous work .
 * While under the possession of the Higurashi family, So'unga was believed to have been the legendary Sword.
 * Sō'unga, along with Mistress Centipede, were the only other characters aside from Kagome and Inuyasha to travel from the Present to the Feudal Era, as well as the only antagonists.
 * 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 bore many similarities to the fictional sword Soul Edge from the  series by Namco. Both were very powerful and sentient swords that were once wielded by heroic leaders (Tōga and Algol, respectively), possessed a malevolent spirit (Sō'unga being the sword itself and Soul Edge's being Inferno) and had the ability to brainwash and manipulate their hosts and both craft armor for them while doing so. In times of desperation, both could 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 literally, being left handed with their swords. Lastly, both had also possessed the main protagonist of the series and mutated their arms to a certain degree. They had also used similar weapons and fighting styles.
 * Sō'unga had 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 was accurate with how old the sword itself was.