Tenseiga

Sesshōmaru's sword, which cannot cut the living but can bring the dead back.

Tenseiga is the sword Sesshōmaru carries through the main story, first wielded by his father Tōga. It began as part of Tessaiga, the sword the swordsmith Tōtōsai made from Tōga's fang, and was split off to hold a deadly technique. It cannot harm humans, but a compassionate wielder can see and cut down beings from the underworld and bring up to a hundred of the dead back to life with one swing.

Quick Facts

Wielder: Sesshōmaru (originally Tōga)
Forged by: Tōtōsai

Facts from the source article.

Background

Sesshōmaru at first despised it as useless in battle and went after his younger brother Inuyasha's Tessaiga instead. His contempt for humans kept him from using Tenseiga until he showed compassion for a human child, Rin, whom he revived with it after she was killed by Kōga's wolves; he later revived his servant Jaken with it too.

The sword has some will of its own and at times urges Sesshōmaru to use it. It can only fully restore someone whose head and body are largely intact.