1x13 - InuYasha: The Final Act

Shishinki, the Ogre of Death, appears before Sesshomaru, telling him that Inuyasha and Sesshomaru's father stole his Meido Zangetsuha in a battle. Shishinki says that Tenseiga is an incomplete sword that is merely a cast-off piece of Tetsusaiga, and that Sesshomaru will never be able to perfect Meido Zangetsuha. Then, a secret involving Inuyasha's Tetsusaiga is revealed!