Inuyasha the Movie 3: Swords of an Honorable Ruler

Tōga denied Sesshomaru's request for ownership of Tessaiga and So'unga two centuries before the main story. He believed Sesshomaru was not worthy of wielding these powerful blades, which were legendary treasures passed down through the dog-demon bloodline. This denial became the source of the ongoing rift between the two brothers, as Sesshomaru resented that his younger half-brother Inuyasha inherited Tessaiga while he received nothing.

Takemaru of Setsuna is the army leader who leads the attack on the mansion where Izayoi is giving birth to Inuyasha. He is secretly in love with Izayoi and cannot accept that she has chosen the dog-demon Tōga over him. Out of jealousy and rage, Takemaru arranges her assassination by stabbing her and setting the mansion ablaze, intending to eliminate both her and the newborn.

So'unga is one of the three legendary swords forged by the blacksmith Tōtōsai for the great dog-demon Tōga. It is the most powerful and dangerous of the three blades, containing an ancient evil demon within its structure. When wielded, the demon can possess the user and unleash devastating attacks like the Dragon Twister, making the sword a threat to all existence if not controlled.

After Kagome's family uncovers So'unga in the modern world, the sheath-conscience named Saya warns that Inuyasha must master the sword before it threatens the world. When Inuyasha grips the blade, So'unga's ancient demon takes control of his arm and drags him back to the Feudal era. The possession amplifies his demonic nature, forcing him to attack his own friends until Kagome uses the Beads of Subjugation to break the sword's hold.

Tōtōsai reveals that the only way to destroy So'unga and Takemaru is to fuse Tessaiga's Backlash Wave with Sesshomaru's Dragon Strike. The Backlash Wave is Tessaiga's defensive technique that reflects attacks, while Dragon Strike is Sesshomaru's sword technique using his own blade Tokijin. When combined, these complementary powers create an attack strong enough to destroy So'unga's physical form and send it into the netherworld.

Takemaru retrieves Sesshomaru's severed arm from two centuries earlier and uses it along with So'unga to rebirth himself into a new body. Tōga's spirit reveals that So'unga houses an ancient evil demon, and this demon's power allows Takemaru to resurrect himself. He then swears vengeance on Tōga by killing his sons Inuyasha and Sesshomaru.

Inuyasha initially refuses to cooperate with his older brother Sesshomaru, insisting they could never stand together. Their relationship has been antagonistic since childhood, fueled by Sesshomaru's jealousy over their father Tōga choosing Inuyasha to inherit Tessaiga while denying Sesshomaru any of the legendary blades. Only the urgent threat of world destruction forces Inuyasha to overcome this rivalry and combine powers with his brother.

Tenseiga is the third sword in Tōga's legendary trio, distinct from Tessaiga and So'unga. Unlike the other two blades designed for combat, Tenseiga is a life-saving sword with the power to revive the dead. Tōga uses it to bring Izayoi back from death's brink after Takemaru stabs her, allowing her to escape with the newborn Inuyasha.

Tōga possessed three legendary swords forged by Tōtōsai: Tessaiga (the Fang), So'unga (the Sound), and Tenseiga (the Heaven). Tessaiga was passed to Inuyasha as a weapon against demons. So'unga, the most dangerous, contains an ancient evil and was sealed away. Tenseiga serves as the life-giving blade with resurrection powers. Each sword represents a different aspect of the dog-demon legacy.

After Inuyasha and Sesshomaru combine their attacks to destroy So'unga's physical body, the sword is hurled into the netherworld through the portal Takemaru opened. The portal then closes, sealing the ancient evil demon within So'unga away from the mortal realm. Tōga's spirit appears one final time to express pride in his sons before the portal shuts.

Kagome carries Saya, the sheath-conscience of So'unga, into the Feudal era because Saya serves as the key to understanding and potentially stopping the sword's power. As the sword's conscious spirit, Saya holds knowledge about So'unga's true nature and can communicate with those who wield it. Her presence helps the group strategize against the threat.

In the flashback two centuries ago, Tōga wounds Takemaru during his attack on the mansion. During this confrontation, Sesshomaru's arm is severed, likely by Tōga himself or in the chaos of the battle. This severed arm is preserved somehow and later retrieved by Takemaru in the present day, allowing him to use it for his resurrection and rebirth with So'unga.