Bakusaiga

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

History
After shattering Tōkijin in combat with Mōryōmaru and passing Tenseiga's only offensive technique, Meidō Zangetsuha to Inuyasha's Tessaiga Sesshōmaru was left without a fighting weapon beyond his own native abilities. While very few demons stood a realistic chance against an unarmed Sesshōmaru, the growing darkness in the Shikon no Tama allowed a demon called Magatsuhi to escape from the jewel itself. Freed, Magatsuhi assembled a body from parts of Naraku and sought out Sesshōmaru in order to steal the last shard of the jewel from Kohaku, who was traveling with Sesshōmaru. Sesshōmaru accepted the challenge of battle and discovered that Magatsuhi had a poison that could physically damage him. The fight was interrupted by the arrival of Inuyasha, who came to his aid. Sesshōmaru, angered by the implication of weakness inherent in the help and pity that he received from both his own followers and Inuyasha's group, manifested a powerful regenerative ability that was composed of demonic power, and caused by his pure fighting spirit, to force his wounds closed. The battle was again joined, and the use of Tenseiga against the otherworldly spirit of Magatsuhi offered hope of victory, but Sesshōmaru was stabbed straight through his armor and chest by Magatsuhi. While Sesshōmaru was engulfed by a mass of demonic flesh that was controlled by Magatsuhi, Inuyasha made a desperate attempt to free him, but only got caught in the engulfing mass himself. At that point, all parties were shocked to see the demonic mass blown apart by a massive burst of demonic power. The light was so blinding, that it took several seconds for the group to see that the light, was in fact, coming from Sesshōmaru's severed left arm. Sesshōmaru, angry, drove back Magatsuhi's probes. At that moment, the sword smith, Tōtōsai appeared. Sesshōmaru, seeing him and realizing in that instant what was happening, met a massive thrust by Magatsuhi with an even more powerful wave of light — in that instant, within the light, his left arm regenerated and along with it, a sword appeared in his new hand. Bakusaiga was formed from Sesshōmaru's previous left arm that Inuyasha had cut off with his sword.

Magatsuhi attempted to reassemble his body. However, he discovered that pieces of his body that were attacked by the new sword not only got destroyed, but also transmitted the blow that they had received to undamaged parts — his body was destroyed while attempting to reform.

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

Sesshōmaru used Bakusaiga for the rest of the series to battle both Naraku and Magatsuhi. During the final battle against Naraku, the Bakusaiga destroyed most of Naraku's body, and is only with the help of it that Inuyasha destroyed forever his body with Meidō Zangetsuha. In the final chapter, set three years after Naraku's defeat, the Bakusaiga was still carried by Sesshōmaru. It was 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 had 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 power, Bakusaiga's blade glows a vivid emerald-green.

Powers & Abilities

 * Decomposition: Bakusaiga's primary ability was to decompose any organic material that its blade cut. Once cut, the target became "infected" with spontaneous destruction that disintegrated the main body and nullified any regenerative properties that the target may have had. In addition, the remaining destruction would continue to spread to any organic matter that came into physical contact with the original target, and was similar to a virus. It was 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 could also fire off explosive blasts of green-colored yōki that were capable of destroying multitudinous weaker demons with ease. Given that this blast was shown destroying the 1000's of demons that were summoned by Byakuya in a single swing, implied that it was easily more than ten times stronger than the Kaze no Kizu.

What Tōga believed
"So that was Bakusaiga's test. Sesshōmaru had to let go of his attachment to our father's sword."

- Inuyasha's observation

Tōga, Sesshōmaru and Inuyasha's father, believed that Sesshōmaru would be able to surpass the powers of Tessaiga. However, Tōga felt that Sesshōmaru could only achieve this if he 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, and 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 believed that Bakusaiga was pronounced "Bah-koo-sai-ga". However, the "koo" sound was so short that the word came out sounding more like "Baak-sai-ga".
 * When Sesshōmaru first obtained Bakusaiga, he wielded it with his left hand, though his right hand was his dominant hand.
 * It appeared as though Sesshōmaru could control, to a certain degree, what was 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 spread 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 cut himself away from his main body in order to avoid being completely destroyed by it.