The Holy Grail was a Biblical artifact said to have supernatural powers, specifically the ability to grant immortality. However, such a prize came with a catch; the Grail was required to stay within the Temple of the Sun and any attempts to take the Grail off the temple's boundaries would result in the destruction of the Temple.
The Grail gave water extraordinary powers, as water applied to a person's skin could rejuvenate their body and heal serious injuries, such as fatal gunshot wounds.
Anyone who drank water from the True Grail would be granted eternal life. However, their eternal life would last so long as they did not cross the Great Seal of the Temple. Passing the Great Seal would cause the drinker to regain his or her mortality, although that would not destroy the Temple.