Tenseiga

The was a yōkai sword that was owned and wielded by Sesshōmaru throughout the main story, and was originally wielded by his father Tōga. It was revealed that Tōga ordered Tōtōsai to forge this blade by departing the Meidō Zangetsuha from the original Tessaiga so that he'd be in possession of two swords which Tōga could give to his sons, one for each. The sword had a special property in it that it could not harm or kill any living beings however, it was able to cut and badly injure beings from the realm of the dead. In the world of the living, its power lay in the fact that Tenseiga could resurrect the dead and heal the wounded.

Tenseiga allowed a compassionate wielder to see and slay the beings from the Underworld, as well as bringing up to 100 beings back to life in one swing. Additionally, while Tenseiga was unable to harm humans, it was able to kill beings that were not from the world of the living, such as spirits, hellhounds, the undead, and other creatures that were native to the Netherworld.

Tenseiga had a slight sentience, as could be seen at various points where it was communicating with Sesshōmaru, and expressed to him things such as a desire to be drawn or prompted him toward certain actions for the benefit of others.

History
The Tenseiga was created by the swordsmith Tōtōsai from the fangs of Tōga, an Inu-Daiyōkai who was said to have been the most powerful yōkai back then and the father of both Inuyasha and Sesshōmaru. The Tenseiga had the power to kill beings of the next world such as pall-bearers who took away the souls of the dead, which allowed the Tenseiga to revive the dead. Tenseiga could revive one hundred people with one swing, the opposite of Tessaiga's ability to slay one hundred yōkai with one swing. After the dog lord's death, Sesshōmaru was left with the Tenseiga.

Originally, Sesshōmaru did not wish to have Tenseiga, because it could not be used as a weapon, and he began to search for his father's other sword, the legendary "Tessaiga". However, the Tessaiga was left to his younger brother, Inuyasha, and had a barrier which prevented him from using it, which forcing Sesshōmaru to look for another, alternative weapon. Sesshōmaru hated humans, and this prevented him from being able to use Tenseiga. It wasn't until he showed compassion and sorrow for the human child, that it finally allowed him to use it’s power for his own gains, such as reviving the head of the yōkai, Goshinki so that his fangs could be used to create the sword, Tōkijin, or when the sword compelled Sesshōmaru to use it for a good cause. Sesshōmaru did not fully understand the behavior of the sword, but he kept it by his side, for it almost always had some unforeseen benefit.

Sesshōmaru's human companion, a child named Rin, was brought back to life with the Tenseiga after she was killed by the demon wolves of Kōga. Jaken, Sesshōmaru's assistant, was also revived by Tenseiga, as well after Kaijinbō killed him while under the influence of Tōkijin.

Tenseiga, interestingly, easily defeated the leader of the Panther Devas, the Panther King, who became an undead after he was revived with the souls of his lieutenants (who had labored to complete the task of reviving him) and 3 shards of the Shikon no Tama. Using the sword, Sesshōmaru restored to life the lesser cats, and thus took the Panther King's life essence away. Inuyasha was then able to finish the job with the Tessaiga and put an end to the renewed war between the Cats and the Dogs.

Since Tenseiga could harm the beings of the Netherworld, the guardians of the gate that separated the Living World from the Dead, Gozu and Mezu, would not attack anyone who wielded the Tenseiga, and thus allowed them to pass through the gate unharmed. However, Tenseiga's power had distinct limitations; in order to fully resurrect a person, their head and body must be relatively intact; resurrecting a decapitated head would restore it to life, but it was incapable of functioning properly without a body.

When Inuyasha and Sesshōmaru's father was still alive, he saved Inuyasha's mother, Izayoi with Tenseiga. Subsequently, as the events developed, Tenseiga and Tessaiga, and in turn, Sesshōmaru and Inuyasha, combined their power to utterly destroy Sō'unga, the Sword of World Conquest, which was more powerful than either of the other two swords alone and had a mind of its own that was bent on destruction.

Reforged into a weapon
Tenseiga was later reforged by Tōtōsai, after Tōkijin was broken, which unlocked the Meidō Zangetsuha, a powerful technique which sent enemies directly to the Netherworld. Thus, Sesshōmaru finally accepted its potential as a weapon. When the Meidō Zangetsuha was used for the first time, it was shaped like a crescent moon. Tōtōsai explained that as Sesshōmaru gained strength, the Meidō would become a full circle that would send the entire body of an enemy into Hell. Sesshōmaru then went on a quest to complete and master his new technique.

As Sesshōmaru trained his Meidō Zangetsuha, the crescent moon shape got bigger, but it did not take the shape of a circle. To solve that problem, Sesshōmaru visited his Mother to ask if his father had told her anything about strengthening the Tenseiga. This led to Sesshōmaru going to Hell to rescue Rin from a dead soul. He found Rin dead and tried to use Tenseiga to revive her, and only found out that the Tenseiga could only revive a person once. Sesshōmaru's grief over losing Rin and his realization that she had died in exchange for Tenseiga to become stronger meant nothing at the cost of her life. This realization strengthened the Tenseiga and Sesshōmaru used it to purify the souls of Hell and created a path to the living world. The Meidō became closer to an oval shape and grew larger. Sesshōmaru's mother revealed that he must have a compassionate heart, which was what allowed the Tenseiga to grow stronger. Sesshōmaru's mother used her Meidō Stone to revive Rin to make her son happy.

Continuing his quest, Sesshōmaru runs into Shishinki, an old enemy of his father's and discovers that Tessaiga cast off Tenseiga to contain the Meidō Zangetsuha technique. During Sesshōmaru's battle with Shishinki, Inuyasha came to his aid and in the presence of Tessaiga, the Tenseiga resonated. Sesshōmaru used the Meidō Zangetsuha and it became a full circle this time, and sent Shishinki's entire body into Hell. Sesshōmaru confronted Tōtōsai about the Tenseiga (going as far as to fire a full Meidō at him), and he discovered that his father intended for him to master the Meidō Zangetsuha and then allow Tessaiga to reabsorb Tenseiga to gain the completed technique.

Sesshōmaru, in anger of his father's supposed betrayal, fell into another trap by Naraku yet again to battle Inuyasha (Although Sesshōmaru seemed to realize that it was a trap). Naraku put a fragment of Kanna's Mirror on the Tenseiga which allowed the sword to steal all of Tessaiga's abilities. Sesshōmaru used the newly empowered Tenseiga to battle Inuyasha and the powerless Tessaiga to see if Inuyasha was truly the Tessaiga's real heir. When Inuyasha defeated the Meidō Zangetsuha and the stolen powers of his own sword, Sesshōmaru broke Tenseiga on Tessaiga which not only restored its powers but granted Tessaiga the Meidō Zangetsuha, just as their father had intended.

When Tenseiga was later revealed to still be intact with its original powers, Sesshōmaru decided to keep the sword. Later, Tōtōsai revealed that Tōga set all of this up to force Sesshōmaru to lose his desire for Tessaiga and grant him his very own sword that was created from his own power, the Bakusaiga.

Teaching compassion
Sesshōmaru despised humans and cared very little about others in general. Tōga knew of this arrogant, egoistic attitude of his first son and hoped to teach him compassion by giving him the Tenseiga. It was also because of this behavior that Sesshōmaru showed that Tōga refused to pass down the Tessaiga to his older son. Since Tenseiga could not be used by someone without compassion, he hoped that Sesshōmaru would learn compassion in order to use the sword.

It had been suspected that Tōga left the Tenseiga to Sesshōmaru for a reason, but the reason was never fully explained until near the end of the series. According to Tōtōsai, Sesshōmaru was given Tenseiga because it could protect him from Tessaiga while it could not be used to harm Inuyasha, in order to teach them to get along. Tenseiga also protected Sesshōmaru several times when his life was in danger from sources other than Tessaiga, notably during his fight with Mōryōmaru. Mōryōmaru taunted Sesshōmaru with Kagura's death, called her worthless, and said that her death was for nothing. Sesshōmaru acted out of anger and sorrow on Kagura's behalf, attacked Mōryōmaru, and tried to break Mōryōmaru's armor with Tōkijin. While it cost him Tōkijin, and Mōryōmaru tried to crush him with the adamant shards that he had stolen from Inuyasha, Tenseiga erected a barrier around Sesshōmaru, which safeguarded his life.

Sesshōmaru's sorrow and anger because of Kagura's death earned him the right to wield Tenseiga as a weapon when Tōtōsai arrived. Tenseiga was reforged and its offensive capability, the Meidō Zangetsuha was unlocked. To strengthen the Meidō Zangetsuha, his father set up the training method that would teach Sesshōmaru the meaning of losing someone close to him and have a compassionate heart. Sesshōmaru's mother summoned Rin's soul from Hell in order to make her son happy, after seeing his pain and Jaken's tears on his master's behalf.

Powers & Abilities

 * Resurrection: The Tenseiga could restore the life of the recently deceased by destroying the pall-bearers who took the souls of the dead. However, it required a relatively-intact body to work and only worked once as not to grant eternal life. If done to a dismembered body part, it will restore the body part to how it was when alive, but it will die again shortly afterwards. This was shown when it was used to revived Goshinki's head to restore his fangs. Although the Tenseiga could not harm what belonged to the Living World, its healing powers allowed it to slay the undead and spirits such Magatsuhi, the evil will of the Shikon Jewel.
 * Purification: After Rin's second death, Sesshōmaru showed compassion to the dead souls in the Underworld, and asked: "You all wished to be saved too, is that it?", and used the Tenseiga to purify them of their sins.
 * Protective Barrier: The Tenseiga had the ability to erect a protective barrier around its master if his life was in danger and transported him across vast distances to safety.
 * Access to the Netherworld: The Tenseiga granted whoever wielded it access into the Netherworld through the Border of the Afterlife. Proof of this was when Gozu and Mezu, the guardians of the Netherworld, allowed Sesshōmaru to enter after he drew the Tenseiga.

Former techniques

 * This technique created a path that sent anyone who was hit with it directly to the Netherworld. The technique originally belonged to the Tessaiga after absorbing it from Shishinki. However, Tōga passed it on to Sesshōmaru in the form of Tenseiga in order to teach him the value of compassion. Originally, the Meidō Zangetsuha manifested as a massive black circle that would send an opponent's entire body and soul to the Netherworld, and left no trace behind. However, when Sesshōmaru first obtained the technique, the Meidō Zangetsuha took the form of a black crescent portal that only sent a part of a person into the Netherworld along with their soul; stronger opponents were thus unaffected by the incomplete Meidō. As Sesshōmaru's compassion grew, the Meidō Zangetsuha was strengthened until it became a perfect circle.

Trivia

 * Tōtōsai first wanted to name the sword "The Coffin Cheater", but called it Tenseiga to be more classy.
 * Much like the Tessaiga, Tenseiga could be sentient. As it sometimes requested to be drawn, or in 3000 Leagues in Search of Father, Tenseiga told Sesshōmaru to save the life of Kanta's Father.
 * It was revealed by Sesshōmaru's mother that Tenseiga could not bring the same person back more than once, as shown when Sesshōmaru tried to revive Rin with Tenseiga after she was brought back from the Netherworld.
 * The name "Tenseiga" is derived from "Tensei", the Japanese word for reincarnation, which referred to the weapon's ability to resurrect people.
 * Tenseiga had the ability to summon its sword smith, Tōtōsai, as seen in the 3rd Episode of The Final Act.
 * While Sesshōmaru's strongest anime-only and movie-only attack Sōryūha was a technique which he usually performed with his Tōkijin, Sesshōmaru was shown using Sōryūha with his Tenseiga in the third InuYasha movie.
 * Tenseiga's Meidō Zangetsuha's could only reach full power if it was close to Tessaiga.
 * Rumiko Takahashi said it wasn't able or was not that easy to mend Sesshōmaru's left arm, since it was cut off with Tessaiga.