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. And it should be noted, that the Tenseiga has a mind of its own, as at various points, it has been seen talking to Sesshomaru by telling him things like wishing to be drawn or giving him advice. The same thing goes for Inuyasha's Tessaiga.

History
The Tenseiga was created by the swordsmith Totosai from a fang of the daiyokai, the Great Dog Demon and father of Inuyasha and Sesshomaru. 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 the Tessaiga ablitiy to slay 100 yokai with one swing. After the dog lord's death Sesshomaru was left the Tenseiga.

Originally Sesshomaru 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. Sesshomaru 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, Sesshomaru started to value the Tenseiga, no longer feeling it was completely useless.

Tenseiga has been used sparingly, mostly when Sesshomaru 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 Tokijin; or when the sword compels Sesshomaru to use it for a good cause. Sesshomaru does not fully understand the behavior of the sword but he keeps it by his side for it almost always has some unforseen benefit.

Sesshomaru'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, Sesshomaru's assistant, is also revived by Tenseiga as well after Kaijinbo kills him while under the influence Tokijin.

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 laboured to complete the task and 3 pieces of the Shikon Jewel. Using the sword, Sesshomaru 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 Sesshomaru's father was still alive, he saved Inuyasha's mother Izayoi with Tenseiga. Subsequently, as the events developed, Tenseiga and Tessaiga, and in turn Sesshomaru 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 Totosai, after Tokijin is broken, which unlocks the Meidou Zangetsuha, a powerful technique which sends enemies directly to Hell. Thus, Sesshomaru finally accepts its potential as a weapon. When the Meidou Zangetsuha was used for the first time it was shaped like a crescent moon. Totosai explains that as Sesshomaru gains strength the Meidou will become a full circle, that will send the entire body of an enemy into hell. Sesshomaru then goes on a quest to complete his new technique.

As Sesshomaru trained his Meidou Zangetsuha, the cresent moon shape got bigger, but it did not take the shape of a circle, so to solve this problem Sesshomaru 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. Sesshomaru'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 Sesshomaru used it to purify the souls of hell and create a path to the living world. The Meidou became closer to an oval shape and grew larger. Sesshomaru's mother reveals that he must have a compassionate heart, which was what allowed the Tenseiga to grow stronger. Sesshomaru's mother used her Meidou Seki to revive Rin to make her son happy.

Continuing his quest Sesshomaru runs into Shishinki, an old enemy of his father's and discovers that Tenseiga cast off Tessaiga to contain the Meidou Zangetsuha technique. During Sesshomaru's battle with Shishinki, Inuyasha comes to his aid and in the presence of Tessaiga, the Tenseiga resonates. Sesshomaru uses the Meidou Zangetsuha and it becomes a full circle this time, and sends Shishinki's entire body into hell. Sesshomaru confronts Totosai about the Tenseiga (going as far as to fire a full Meidou at him), and he discovers that his father intended for him to master the Meidou Zangetsuha and then allow Tessaiga to reabsorb Tenseiga to gain the completed technique.

Sesshomaru, in anger of his father's supposed betrayal falls in another tramp by Naraku yet again to battle Inuyasha. He puts melts a fragment of Kanna's mirror on the Tenseiga which allows the sword to steal all of Tessaiga's ablities. Sessohmaru uses the newly empowered Tenseiga to battle Inuyasha and the powerless Tessaiga to see if Inuyasha was truly the Tessaiga's real heritor. When Inuyasha defeats the Meidou Zangetsuha and the stolen powers of his own sword, Sesshomaru breaks Tenseiga on Tessaiga which not only restores its powers, but grants Tessaiga the Meidou Zangetsuha just as their father intended.

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

Teaching Compassion
Sesshomaru hated humans and cared very little about others and the Inu no Taishou knew of this and hoped to teach him compassion by giving him Tenseiga. It was also because of this behaviour Sesshomaru shows that Inu no Taishou refused to pass down the Tessaiga to his older son. Since Tenseiga could not be used by someone without compassion he hoped that Sesshomaru would learn compassion in order to use the sword.

It had been suspected that the Inu no Taishou left the Tenseiga to Sesshomaru for a reason, but the reason was never fully explained until near the end of the series. According to Totosai, Sesshomaru 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 Sesshomaru several times when his life was in danger from sources other than Tessaiga, notably his fight Mouryomaru. Mouryomaru taunted Sesshomaru with Kagura's death, calling her worthless and saying her death was for nothing. Sesshomaru acted out of anger and sorrow on Kagura's behalf, and attacked Mouryomaru, trying to break Mouryomaru's armor with Tokijin. While it cost him Tokijin, and Mouryomaru tried to crush him with the adamant shards he stole from Inuyasha, Tenseiga erected a barrier around Sesshomaru, safeguarding his life.

His sorrow and anger for Kagura earned him the right to wield Tenseiga as a weapon when Totosai arrives. Tenseiga is reforged and its offensive capability, Meidou Zangetsuha, is unlocked. To strengthen the Meidou his father set up the training method that would teach Sesshomaru the meaning of losing someone close to him and have a compassionate heart. Sesshomaru'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

 * Healing Cut: Restores the life of the dead by cutting the pall-bearers who take the souls of the dead. It was also able to cut Magatsuhi, the manifestation of the evil yōkai with whom Midoriko battles inside the jewel. This ability allows the sword to cut things that are not of this world.
 * Save a Hundred Souls with One Swing: This move is only seen in the Final Act in Episode 9 when Sesshomaru enters the Underworld. Because of Rin's death, Sesshomaru shows compassion to the dead souls in the Underworld, asking, "You all wished to be saved too, is that it?" Thus he grants their wish by using Tensaiga to purify their souls, as noted by his mother who looks through the Meido Stone she possesses. This move allows Sesshomaru to increase the size of the move Meido Zangetsuha.


 * Protective barrier: Protects Sesshomaru. The sword acts on its own when his life is in serious danger. Note: Even though Sesshomaru despises the sword, it saves Sesshomaru's life, merely because it is the sword of life.


 * Access to the Netherworld: Grants Sesshomaru access into the Netherworld through the Border of the Afterlife as Tenseiga isn't a blade of this world.

Removed Powers

 * Meidō Zangetsuha (冥道残月破, "Dark Path Waning Moon Crush") is a technique that creates a path that sends anyone who is hit with it directly to hell, killing them without actually harming them. The technique originally belonged to Inu no Taishō's own sword; however, he passed it on to his son Sesshōmaru in order to give him the power to protect, not just to kill. The Meidō Zangetsuha creates a massive black circle that will send a entire body into hell, leaving no trace left behind. However, when Sesshōmaru was first learning to use the technique, the Meidō will take the form of a black cresent moon that only sent a part of a person into hell along with their soul; stronger opponents may be uneffected by the cresent Meidou. As Sesshōmaru gained strength, the Meidō grew until it became a perfect circle.

Trivia

 * Totosai 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 Sesshomaru to save the life of Kanta's father.
 * It is revealed by Sesshōmaru's mother that Tenseiga could only revive someone once. As shown when Sesshomaru 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 Sesshomaru in the Underworld.
 * It could be theorized that Tenseiga probably can't/won't save someone who dies of natural causes (such as old age, heart attack, etc.) or someone who sacrifices themselves or commits suicide as well.