So'unga

The So'unga is a fictional sword from the anime series InuYasha, appearing only in the third movie as its primary villain.

History
Also known as the Sword of Calamity, So'unga was the former property of the great dog general (Inu no Taishou), InuYasha and Sesshomaru's father. Unlike Tessaiga and Tenseiga (InuYasha's and Sesshomaru's swords, respectively), this sword, which resembles a Zweihänder, was not forged by the sword smith, Tōtōsai. Not much is known of So'unga's history other than the dog demon acquired it on his own and it's said that he was the only yōkai of few who could wield it without being possessed by it. Either way its true power can only be used by demons or those with demon blood. (Takemaru was only able to use Sounga’s true power by wielding Sesshomaru’s lost arm)

In InuYasha's timeline, only Sesshomaru is able to use and wield So'unga without being possessed by it by overcoming So'unga with his own demonic power. It was known that Sesshomaru desired the swords Tessaiga and Sounga in his quest for power. After Inu no Taishou died in his attempt to rescue his human wife Izayoi and his son InuYasha, it was clear that he had passed instructions on what to do with his powerful swords to his loyal friends. It was vital that the swords, especially So'unga do not fall into the hands of evil-doers as they would bring untold destruction unto the world if misused.

Myōga (the flea demon), Saya (the spirit of the [sheath] of So'unga) and Tōtōsai (the sword smith who forged Tessaiga and Tenseiga) followed their lord's instructions with Myōga sealing Tessaiga along with their lord's body in the boundary between the living world and the dead and Tōtōsai leaving Tenseiga for Sesshomaru. However, Saya had clearly forgotten Inu no Taishou's instructions on what to do with So'unga and in the end, they threw the sword down the Bone-Eater's Well where it was known that demon remains thrown down the well mysteriously vanished. It was decided that Saya would seal the sword's malicious powers for at least seven hundred years. Thus, it ended up in Kagome's shrine.

So'unga attempts to possess anyone who tries to wield it. After possessing InuYasha and unleashing the terrible power of Goukuryuuha [dragon twister], InuYasha sought solitude to keep his friends safe. Soon after, Sesshomaru showed up and attempted to claim the sword. Later the sword claimed Sesshomaru's lost arm in order to unleash the totality of its power. So'unga was later defeated by the combined powers of Tessaiga and Tenseiga. Having opened the door to the underworld, when So'unga was defeated, it was sealed in the portal that it itself opened and the threat that it presented was gone for good.

Only one of So'unga's demon attacks is known: Goukuryuuha (Infernal Dragon Wave or Dragon Twister). The attack takes the form of a gigantic purple tornado with its center glowing pink/light purple.

Information from InuYasha the Movie 3: Swords of an Honorable Ruler reveals that So'unga's has the special ability to resurrect both human and yōkai. All enemies slain by So'unga become its zombie slaves.

Goukuryuuha
The Infernal Dragon Wave is the ultimate attack of So'unga, which summons a dragon (or dragons) of hell, and releasing it as an attack that is more powerful than Tessaiga and Tensiega by themselves. It takes the shape of huge orb of demonic energy, formed in the middle of the rotating sword generating a huge mass of hellishes winds wrapped around it, which is then blasted at the enemy with more concentrated force than an F-5 tornado. Its power of destruction is more than several times more stronger than even the Tessaiga's Bakuryuuha.

A difference of power that can be seen in the movie, as the Backlash Wave could barely withstand, let along repel, the Goukuryuuha. In fact, in some cases, it could not even be formed by Inuyasha due to Sounga's utter power of destruction. However, it is notable that when in close proximity to both the Tessaiga and Tenseiga, its power is weakened. Thus as long as both swords exist, and are within its presense, the sword's attack can never reach its full potential. Some said both Tessaiga and Tenseiga was made to suppress So'unga.

Yet, one can assume that the sword requires its wielder to have some sort of youki, or some relation to yōkai, mainly because of Sounga's advice to Takemaru to travel to the border of this world and the next to retrieve Sesshomaru's lost yōkai arm. Which it seems was necessary for him to attach it to himself, in order for him to be able to summon the dragon of hell and use the Goukuryuuha attack.

Bearing this in mind, then swords like the So'unga, Tessaiga and Tensaiga attack opponents by using their own youki (demonic energy) combined with the youki of the demon who wield it, it is safe to assume that we never really get to see the real power of this sword even in the movie. This conclusion is reached because the only time the Goukuryuuha was seen unleashed was when either it was wielded by InuYasha, Takemaru or So'unga by itself using Sesshomaru's arm. If another like Inu no Taishou, or Sesshomaru himself, were to perform this attack, with their enormous amounts of youki, then it would be capable of causing unparallel amount of destruction.


 * Goukuryuuha is translated as Hell Dragon's Wave. In the English dubbed version and English translated subtitles, it is called Dragon Twister

100 zombies
Anything killed by So'unga comes back as highly poisonous zombies are under So'unga or the wielder of So'unga’s control. The only way to destroy them is to damage them beyond their abilities to regenerate, such as setting fire to their flammable bodies, or use of a holy item (like Kagome's Sacred Arrows). While Tessaiga could take 100 lives away and Tenseiga could bring 100 people back to life, Sounga raises 100 zombies from the underworld. It should be noted, that the zombies being of the underworld, could be cut and killed by the Tenseiga.

Demonic Control
So'unga is said to contain the soul of a horribly evil demon (possibly a dragon from Hell), and has the power to possess and control its user, manipulating them. The sword is sentient, possesing a violent, bloodlusting personality and a desire only for war, destruction, slaughter, and self-preservation. So'unga, in a sense, is not unlike Tōkijin in its powers of possession and their being swords of evil except that it is far stronger than the latter. Only Inu no Taishou, Sesshomaru and other daiyōkai (possibly) are able to resist this power outright. The worst thing that could have happened was for So'unga to fall into the hands of a human. Humans are (supposedly) the most selfish of creatures and have the deepest desires for power; if a human holds So'unga, he would not stop even for centuries until the world is destroyed in his mad conquest or the user itself is being killed by something.

Hell Spikes
In the movie Sounga had fired several large black spikes from within his armor.

Opening the Underworld
Being the sword of hell, Sounga has the power to open the doors to hell, which would merge with the world of living and cause every living thing in the world to perish.