Seimeikan

Seimeikan (生命館, せいめいくわん, "Life Channel"): The Seimeikan was a special organ located inside Nikosen's body resembling a white root. As long as this organ was intact, Nikosen was 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 youki which allowed Inuyasha to fully control its demonic aura.