Ryūkotsusei

A long time ago, InuYasha's father fought with Ryukotsusei. He at last imprisoned him, but soon after died from the wounds of battle. Later, InuYasha's sword, the Tetsusaiga got heavier and his companion, Myoga the flea told him that the only way to make the Tetsusaiga lighter was to outdo his father. The only way to do this, of course, was to kill Ryukotsusei completely without dying. The group reached the cliff where Ryukotsusei was imprisoned. Unfortunately for InuYasha, Naraku reached the cliff first, and freed the demon. Ryukotsusei was very angry at InuYasha's father, and wanted to kill InuYasha himself as the son of his imprisoner. InuYasha and Ryukotsusei fought, nearly killing InuYasha. At last, InuYasha found a new technique in his sword, the Bakuryuha, or as it is known in the English dub the "Backlash Wave". He used this and killed Ryukotsusei, outdoing his father and making the Tetsusaiga lighter.