The Tetsusaiga is a weapon, a sword which is based on two seperate forms. One which you may have seen inuyasha holding in an image which is the good form of the sword which is a gigantic sword. There is also another form which is a small blade which is the evil form of the sword.

The story behind obtaining the Tetsusaiga, begins with Sesshomaru searching for it, but found inuyasha instead. The key to finding the Tetsusaiga was through Inuyasha. Inuyashas eye opened a portal which they both went through. Inside was the grave of Inuyashas father and the Tetsusaiga was inside the skeleton of Inuyashas father. They found the Tetsusaiga, Sessomaru found it first and was attempting to pull out the Tetsusaiga to obtain it, but failed because he was a full demon. Once Inuyasha and Kagome found it, Kagome told Inuyasha to pull out the Tetsusaiga to hurt Sesshomaru's pride. But Inuyasha also could not pull out the Tetsusaiga either. The two of them started fighting, while Kagome wondered to the Tetsusaiga and pulled it out. They then left the skeleton, and Sesshomaru turned into a giant demon dog. The Tetsusaiga then transformed into the gigantic blade and Inuyasha attacked Sesshomaru and cut off an arm and defeated but did not kill Sesshomaru. Later Sesshomaru tried to destroy the Tetsusaiga but Inuyasha tried to stop him using his Wind Scar but Sesshomaru's Tensusaiga saved him from the attack. Inuyashas father then gacve Sesshomaru the Tensusaiga and Inuyasha the Tetsusaiga and they could not kill each other.