For whatever reason - presumably to protect Izayoi and Inuyasha - Inutaishou battled Ryuukossei, a monstrous purple dragon. Though he was to powerful to kill, Inutaishou managed to seal him in a powerful spell by puncturing his heart with a claw. Ryuukossei was but into a dormant state of sleep, and the injuries Inutaishou received from the battle ultimately killed him. His body was 'mightier than iron', and he had the ability to spit huge balls of energy at his opponents.

When Inuyasha comes to him to make his sword lighter, Toutousai sends him to kill Ryuukossei. Because he was still trapped under the spell, all Inuyasha had to do was cut out his heart while he slept. Unfortunately, Naraku overhears and destroys the spell by melting away Inutaishou's claw, leaving Inuyasha with a vengeful dragon to battle. He manages to kill Ryuukossei with a new attack he discovers during the battle, Bakuryuuha.