Bakusaiga

is a sword that was created from Sesshōmaru's own body. The Bakusaiga came from Sesshōmaru's body attached to his finally regenerated arm. It is Sesshōmaru's final signature sword.

History
After shattering the Tōkijin in combat with Mōryōmaru and passing Tenseiga's only offensive technique Meidō Zangetsuha to Inuyasha's Tessaiga Sesshōmaru is left without a fighting weapon beyond his own native abilities. While very few demons stand a realistic chance against an unarmed Sesshōmaru, the growing darkness in the Shikon no Tama allows a demon called Magatsuhi to escape from the jewel itself. Freed, Magatsuhi assembles a body from parts of Naraku and seeks out Sesshōmaru in order to steal the last shard of the jewel from Kohaku, who is traveling with Sesshōmaru. Sesshōmaru accepts the challenge of battle and discovers that Magatsuhi has a poison that can physically damage him. The fight is interrupted by the arrival of Inuyasha, who comes to his aid. Sesshōmaru, angered by the implication of weakness inherent in the help and pity he receives from both his own followers and Inuyasha's group, manifests a powerful regenerative ability composed of demonic power, caused by his pure fighting spirit, to force his wounds closed. The battle is again joined, and the use of Tenseiga against the otherworldly spirit of Magatsuhi offers hope of victory, but Sesshōmaru is stabbed straight through his armor and chest by Magatsuhi. While he is engulfed by a mass of demonic flesh controlled by Magatsuhi, Inuyasha makes a desperate attempt to free him but only gets caught in the engulfing mass himself.

At this point, all parties are shocked to see the demonic mass blown apart by a massive burst of demonic power. The light is so blinding it takes several seconds for the group to see that the light, is in fact, coming from Sesshōmaru's severed arm. Sesshōmaru, angry, drives back Magatsuhi's probes. At this moment the sword smith Tōtōsai appears. Sesshōmaru, seeing him and realizing in that instant what is happening, meets a massive thrust by Magatsuhi with an even more powerful wave of light - in that instant, within the light his arm regenerates and along with it a sword appears in the new hand. Bakusaiga was formed from Sesshōmaru's previous left arm that Inuyasha cut off with his sword.

Magatsuhi attempts to reassemble his body. However, he discovers that pieces of his body attacked by the new sword not only get destroyed, but also transmit the blow they have received to undamaged parts - his body is destroyed attempting to reform.

In the course of the final stages of the battle Tōtōsai reveals that the new sword is called Bakusaiga. It is not an heirloom or crafted sword but is created from Sesshōmaru's own body as a manifestation of his own demonic power, and thus truly his own. His success in abandoning his interest in stealing Tessaiga as a quick path to power resulted in the development of his own inner strength, in the end surpassing even that of his father and the two swords derived from him.

Sesshōmaru uses Bakusaiga for the rest of the series to battle both Naraku and Magatsuhi. During the Final Battle against Naraku, the Bakusaiga destroys most of Naraku's body. In the final chapter, set three years after Naraku's defeat, the Bakusaiga is still carried by Sesshōmaru. It is also noted that Sesshōmaru could have obliterated both Inuyasha and Naraku at various times with the Bakusaiga, but chose not to do so. He began to understand that he could not use such power recklessly, and acted swiftly to save Kagome as well as Rin (as this was why Naraku kidnapped Rin so that if he used Bakusaiga to destroy his body, Bakusaiga's power would have most likely killed her as well).

Physical description
Bakusaiga takes on the form of a katana with a unique design. The backside of its blade features various rune-like lines running along it with a beige hilt featuring the same design. When unleashing its poisonous powers, Bakusaiga's blade glows a vivid emerald-green.

Powers & Abilities

 * Decomposition: Bakusaiga's primary ability is to decompose any organic material its blade cuts. Once cut, the target becomes "infected" with spontaneous destruction that disintegrates the main body and nullifies any regenerative properties the target may have. In addition, the remaining destruction will continue to spread to any organic matter that comes into physical contact with the original target, similar to a virus. It is because of this particular ability that Bakusaiga proved to be a bane to Naraku, as it prevented him from absorbing or reabsorbing yōkai or parts of himself that were cut by the sword without risking being destroyed himself.
 * Yōki Wave: In addition to its decomposition ability, the Bakusaiga can also fire off explosive blasts of green-colored yōki capable of destroying multitudinous weaker demons with ease. Given that this blast was shown destroying the 1000's of demons summoned by Byakuya in a single swing implies that it is easily more than ten times stronger than the Kaze no Kizu. Additionally, Rumiko Takahashi herself stated that Bakusaiga's energy attack, while similar to the Kaze no Kizu, possesses much greater strength and power than Inuyasha's full-powered Bakuryūha.

What the Inu no Taishō believed....
The Inu no Taishō, Sesshōmaru and Inuyasha's father, believed that Sesshōmaru would be able to surpass the powers of Tessaiga. However, the Inu no Taishō felt that Sesshōmaru could only achieve this if Sesshōmaru let go of his grudge against his father's will; he left the Tessaiga to Inuyasha and the Tenseiga, the sword that "will not cut", to Sesshōmaru. Sesshōmaru finally let go of his grudge during his battle with Magatsuhi. From then on, Sesshōmaru had inherited the Great Dog Demon's title; Sesshōmaru gained a sword of his own: Bakusaiga. Bakusaiga matched the personality of its master, Sesshōmaru: the cruel streak of killing thousands at once.

Trivia

 * Some fans mistakenly believe Bakusaiga is pronounced "Bah-koo-sai-ga". However, the "koo" sound is so short that the word comes out sounding more like "Baak-sai-ga".
 * When Sesshōmaru first obtained Bakusaiga, he wielded it with his left hand, though his right hand is his dominant hand.
 * It appears as though Sesshōmaru can control, to a certain degree, what is destroyed by Bakusaiga, as Kohaku remained unharmed despite being in contact with Naraku's Bakusaiga-cut tentacles when they were destroyed.  Likewise, when battling inside Naraku's body, none of Inuyasha's friends were harmed despite repeatedly coming into contact with Naraku's disintegrating body (courtesy of Bakusaiga). It could simply be that it only spreads through attached parts and, while in contact with said parts, they were still separate entities and thus unharmed by it. It's even noted that Naraku cuts himself away from his main body in order to avoid being completely destroyed by it.