The technique of immortality allows the user to extend their life to great periods of time that far out last the normal human lifespan. However, it does not stop the aging process.
Master Tengen's innate technique has allowed them to live since the Nara Period of Japan's history.
Every five hundred years they must reset their body in order to prevent the technique from attempting to change bodies on its own and force evolution. To refresh their body and reset the technique, Tengen must merge with a compatible Star Plasma Vessel.