Seimeikan

The is a special organ located inside Nikosen's body, resembling a white root. As long as this organ is intact, Nikosen is immortal and could heal from any wound. When the organ was severed by Inuyasha, Nikosen died and turned into a pile of wood, and strengthened the Dragon-Scaled Tessaiga with yōki which allowed Inuyasha to fully control its demonic aura.