Toga, an extremely powerful yokai and the father of Inuyasha and Sesshomaru, left one sword to each of his children, and a third one behind for them to deal with. Inuyasha received Tessaiga, Sesshomaru received Tenseiga, and So’unga was the third. Each of the swords contains incredible properties, and Toga chose who received which sword with care.
Inuyasha is an isekai anime where Kagome, a young girl, falls into a well and finds herself in feudal Japan. She must team up with Inuyasha, a half-demon warrior, to recover all of the shards of a power gem, the Shikon jewel. In the process, Inuyasha and Kagome grow close and the relics of his father play a powerful role.
Tessaiga
The Tessaiga was forged from the fang of Toga by a swordsmith named Totosai and was known as the sword of destruction. It was rumored to have felled 100 in a single stroke. When not in its activated form it resembles a damaged katana and can do very little damage. However, when it is activated by Inuyasha using it to protect humans, it transforms into a massive weapon similar to a dadao saber. In this enhanced form it also more closely resembles the fang it was created from.
The sword contains a few special properties, the attack that can be used to fell 100 in a single stroke for example, as well as the ability to absorb abilities from demons that it defeats. While a human cannot activate the Tessaiga, due to a lack of yoki or demonic energy, they can still benefit from its defensive powers as evidenced by Kagome defending herself both from Sesshomaru and the spider-head demon. This is also shown by Inuyasha himself being unable to fully utilize the sword on the night of the new moon in his human form. The final ability of the Tessaiga is that it contains the power to keep Inuyasha’s demon half under control.
Unfortunately for Inuyasha, Tessaiga is eventually broken. Long after this fateful moment, Totasai was able to reforge it using a fang from Inuyasha himself. This made the sword depend upon his power instead of his father’s. The sword was far sturdier, but heavier after this change.
Tenseiga
Where the Tessaiga is the sword of destruction, the Tenseiga is the sword of life. It is rumored to have resurrected 100 with a single stroke. It cannot harm the living, instead when used upon a living being it heals them. It can however harm beings from the realm of the dead.
This sword was also crafted by Totosai from a fang of Toga. However, it was later revealed that the Tenseiga and the Tessaiga were originally a single sword. Toga had them split into two after the Tessaiga stole the ability to heal and resurrect the dead as he felt that technique was too dangerous to pass along to Inuyasha. This sword was instead passed along to Sesshomaru who for a while felt that it was useless since it could not be used against the living. It did eventually acquire the power to send an enemy to the netherworld at which point he accepted it as a weapon.
So’unga
The So’unga was known as the sword of calamity. It was rumored to be able to summon 100 dead souls in one swing. On its own, it was stronger than both the Tessaiga and the Tenseiga, though together the two could defeat it. This sword was not created by Toga, nor from a part of his body. In fact, its origin is so shrouded in mystery that none are sure how or why Toga originally acquired it. One theory is that he acquired it to quell the conflict between humans and demons, another says that it contains the soul of an ancient and evil dragon.
Also, unlike the other two swords, the So’unga was decidedly evil and would attempt to claim the minds of its wielders. Humans were nearly immediately possessed by the blade and even powerful demons were overcome. Toga himself was the only demon known to have been able to control the sword without becoming corrupted. Even Inuyasha was overcome by it.
Each of the blades left behind by Toga were extremely powerful and useful in completely different contexts. The Tenseiga and Tessaiga were forged specifically for his sons and with the intention of teaching them unique lessons. Sesshomaru he hoped would learn compassion and quell some of his jealousy towards Inuyasha. Inuyasha, he hoped, would come to love his human half and protect humans as Toga himself had hoped to do. So’unga was something that he didn’t mean to leave to them but had hoped that Inuyasha would become powerful enough to control. In the end, his children together were powerful enough to defeat it for all time, in the process fully accepting the lessons that their father tried to impart.