So'unga

Former property of the great dog general (Inu no Taishou), InuYasha and Sesshomaru's father. Unlike Tessaiga and Tenseiga this sword was not forged by the sword smith, Totosai. Not much is known of Sounga'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. Some fans speculate that that it was forged from the fang of the dog general or possibly his father, but by a different sword smith. Either way it is somehow related to his family since only those of his blood or those carry organs of his blood such as the way 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 Sounga without being possessed by it by overcoming Sounga with his own demonic power. It was known that Sesshomaru desired the swords Tessaiga and Sounga in his quest for power. After Inu no Taisho 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 Sounga do not fall into the hands of evil-doers as they would bring untold destruction unto the world if misused.

Myoga (the flea demon), Saya (the spirit of the sheath of Sounga) and Totosai (the sword smith who forged Tessaiga and Tenseiga) followed their lord's instructions with Myoga sealing Tessaiga along with their lord's body in the boundary between the living world and the dead and Totosai leaving Tenseiga for Sesshomaru. However, Saya had clearly forgotten Inu no Taisho's instructions on what to do with Sounga 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.

Sounga attempts to possess anyone who tries to wield it. After possessing InuYasha and unleashing the terrible power of Gokuryuuha, 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. Sounga was later defeated by the combined powers of Tessaiga and Tenseiga. Having opened the door to the underworld, when Sounga 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 Sounga's demon attacks is known: Gokuryuuha (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 Sounga's has the special ability to resurrect both human and yōkai. All enemies slain by Sounga become its zombie slaves.

Goukuryuuha
The ultimate attack of Sounga. An attack from a dragon of hell, it is more powerful than Tessaiga and Tensiega by themselves. A huge orb is formed at the end of the sword rotating a huge mass of hell wind wrapped around it, which is then blasted at its enemy. Its power of destruction is beyond Tessaiga's Bakuryuha and it can be seen in the movie that the Backlash Wave could barely withstand Gokuryuuha and in some cases, it could not even be formed due to Sounga's utter power of destruction. It is noted that close proximity to Tessaiga and Tenseiga weakens its power, and thus as long as the two swords exist it can never reach its full potential.

Bearing in mind that swords like the Sounga, Tessaiga and Tensaiga attack opponents by using their own yoki (demonic energy) combined with the yoki of the men 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 because the only time Goukuryuuha was seen unleashed was when either it was wielded by InuYasha, Takemaru or Sounga by itself. If someone like Inu no Taisho or Sesshomaru performed this attack, due to their enormous amount of yoki, the attack 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 Sounga comes back as zombies are under Sounga or the wielder of Sounga’s control. The only way to destroy them is to damage them beyond their abilities to regenerate. 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
Sounga has the power to possess and control its user, manipulating them. Sounga, in a sense, is not unlike Tokijin in its powers of possession and their being swords of evil except that it is far stronger than the latter. Only Inu no Taisho and Sesshomaru are able to resist this power. The worst thing that could have happened was for Sounga to fall into the hands of a human. Humans are the most selfish of creatures and have the deepest desires; if a human holds Sounga, he would not stop until the world is destroyed in his mad conquest.

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.