Tenseiga

The is a sword that belongs to Sesshōmaru, and was once wielded by his Father. The sword has a special property in that it cannot harm any living beings, which is ironic for a sword. However its power lies in the fact that it can bring the recently dead back to life as well as healing the wounded. The sword was once part of Inuyasha's Tessaiga.

Tenseiga allows a powerful 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 is unable to harm humans, it is able to cut through beings that are not from the world of the living, such as the living dead.

Tenseiga has a mind of its own, as can be seen at various points where it is communicating with Sesshōmaru, expressing to him things such as a desire to be drawn or giving him advice. The same thing goes for Inuyasha's Tessaiga.

History
The Tenseiga was created by the swordsmith Tōtōsai from a fang of the daiyōkai, the Great Dog Demon and father of Inuyasha and Sesshōmaru. The Tenseiga has the power to cut beings of the next world such as pall-bearers who take away the souls of the dead, this allows the Tenseiga to revive the dead. Tenseiga can revive 100 people with one swing, the opposite of Tessaiga's ability to slay 100 yōkai with one swing. After the dog lord's death Sesshōmaru was left 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 Tessaiga. However the Tessaiga was left to his younger brother Inuyasha and had a barrier which prevented him from using it, forcing him to look for another 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 Rin, that he was able to use it. After using its power, Sesshōmaru started to value the Tenseiga, no longer feeling it was completely useless.

Tenseiga has been used sparingly, mostly when Sesshōmaru feels like using its 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 compels Sesshōmaru to use it for a good cause. Sesshōmaru does not fully understand the behavior of the sword but he keeps it by his side for it almost always has some unforeseen benefit.

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

Tenseiga, interestingly, easily defeats the leader of the panther devas, the Panther King, who becomes undead when he is revived with the souls of his lieutenants (who had labored to complete the task of reviving him) and 3 shards of the Shikon Jewel. Using the sword, Sesshōmaru restored the life back of the lesser cats, thus taking the Panther King's life essence away. Inuyasha is 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 can harm the beings of the netherworld, the guardians of the gate that separates the Living World from the Dead, Gozu and Mezu, will not attack anyone who wields the Tenseiga, thus allowing them to pass the gate unharmed. However, Tenseiga is not all-powerful as apparently it is not able to save beings who are extremely injured or destroyed such as Kagura, the wind sorceress created from Naraku and wind. The sword is either unable or unwilling to help such beings that do not possess a soul to begin with. Alternately, Tenseiga may require a more or less intact body to revive, which was not available as Kagura was completely disintegrated by Naraku's miasma.

In the third theatrical release of the InuYasha series, it is shown that 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, combine their power to utterly destroy Sō'unga, the sword of world conquest, which is more powerful than either of the other two swords alone and has a mind of its own that is bent on destruction.

Reforged Into A Weapon
Tenseiga is later reforged by Tōtōsai, after Tōkijin is broken, which unlocks the Meidō Zangetsuha, a powerful technique which sends enemies directly to the Netherworld. Thus, Sesshōmaru finally accepts its potential as a weapon. When the Meidō Zangetsuha was used for the first time it was shaped like a crescent moon. Tōtōsai explains that as Sesshōmaru gains strength the Meidō will become a full circle, that will send the entire body of an enemy into hell. Sesshōmaru then goes on a quest to complete 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, so to solve this problem Sesshōmaru visited his mother to ask if his father told her anything about strengthening the Tenseiga. This led to Sesshomara going to hell to rescue Rin from a dead soul. He found Rin dead and tried to use Tenseiga to revive her only find out that the Tenseiga can only revive a person once. Sesshōmaru's grief over losing Rin and realizing that she 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 create a path to the living world. The Meidō became closer to an oval shape and grew larger. Sesshōmaru's mother reveals 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 Tenseiga cast off Tessaiga to contain the Meidō Zangetsuha technique. During Sesshōmaru's battle with Shishinki, Inuyasha comes to his aid and in the presence of Tessaiga, the Tenseiga resonates. Sesshōmaru uses the Meidō Zangetsuha and it becomes a full circle this time, and sends Shishinki's entire body into hell. Sesshōmaru confronts Tōtōsai about the Tenseiga (going as far as to fire a full Meidō at him), and he discovers 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, falls in another trap by Naraku yet again to battle Inuyasha (although Sesshōmaru seems to realize that it is a trap). He puts a fragment of Kanna's mirror on the Tenseiga which allows the sword to steal all of Tessaiga's abilities. Sesshōmaru uses the newly empowered Tenseiga to battle Inuyasha and the powerless Tessaiga to see if Inuyasha was truly the Tessaiga's real heir. When Inuyasha defeats the Meidō Zangetsuha and the stolen powers of his own sword, Sesshōmaru breaks Tenseiga on Tessaiga which not only restores its powers, but grants Tessaiga the Meidō Zangetsuha just as their father intended.

When Tenseiga was later revealed to still be intact with its original powers, Sesshōmaru decided to keep the sword. Later Tōtōsai reveals that the Inu no Taishō set all of this up to force Sesshōmaru to lose his desire for Tessaiga and grant him his very own sword created from his own power, the Bakusaiga.

Teaching Compassion
Sesshōmaru hated humans and cared very little about others and the Inu no Taishō knew of this and hoped to teach him compassion by giving him Tenseiga. It was also because of this behavior Sesshōmaru shows that the Inu no Taishō 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 the Inu no Taishō 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, calling her worthless and saying her death was for nothing. Sesshōmaru acted out of anger and sorrow on Kagura's behalf, and attacked Mōryōmaru, trying to break Mōryōmaru's armor with Tōkijin. While it cost him Tōkijin, and Mōryōmaru tried to crush him with the adamant shards he stole from Inuyasha, Tenseiga erected a barrier around Sesshōmaru, safeguarding his life.

His sorrow and anger because of Kagura's death earned him the right to wield Tenseiga as a weapon when Tōtōsai arrives. Tenseiga is reforged and its offensive capability, Meidō Zangetsuha is unlocked. To strengthen the Meidō 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 summons 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.

Abilities

 * Resurrection: The Tenseiga could restore the life of the recently deceased by destroying the pall-bearers who take the souls of the dead; however, it required a relatively-intact body to work and only worked once as not to grant eternal life. 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, asking, "You all wished to be saved too, is that it?", and used the Tenseiga to purify them of their sins.
 * Teleportation: The Tenseiga had the ability to transport its wielder across vast distances if their life was in danger.
 * 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

 * Meidō Zangetsuha (冥道残月破, "Dark Path of the Dawn's Moon Wave"). 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, the Inu no Taishou passed it on to Sesshōmaru in the form of Tenseiga in order to teach him the value of compassion. Oriignally, the Meidō Zangetsuha manifested as a massive black circle that would send an opponent's entire body and soul to the Netherworld, leaving 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 is revealed by Sesshōmaru's mother that Tenseiga cannot bring the same person back more than once. As shown when Sesshōmaru tried to revive Rin after she was brought back from the netherworld.
 * The Japanese word "Tensei" is a word for reincarnation, which actually fits in with the weapons' ability to resurrect people.
 * Even though it's power to save 100 lives in one sweep has been stated many a time throughout the series, the name has not be revealed, but the move has been seen in Sesshōmaru in the Underworld.
 * It could be theorized that Tenseiga probaTotosai talks to Sesshomaru.jpg can't/won't save someone who dies of natural causes (such as old age) or someone who sacrifices themselves or commits suicide as well.
 * Tenseiga has the ability to summon its sword smith Tōtōsai as seen in the 3rd episode of The Final Act. Presumably Tessaiga can also do this.
 * Tenseiga's Meidō Zangetsuha's power can be significantly increased if it is close to Tessaiga.