A goblet noted to have been touched by the blood of Christ that can grant any wish the holder desires.The
Einzberns, Matous and Tohsakas thanks to their teachings had created the Holy Grail War where seven servants by 7 masters fight each other till only 1 survives. Sadly the holy grail can be corrupted and was in the past by an avenger that held the name Angra Mainyu but I'm getting ahead of myself, let us just use the complete version that was not tainted.
All who desire this Grail rather they be human, God, devil or Servant Why do you desire for this prestige item and what is your wish? Do you desire it's destruction? If so be prepared to suffer the eternal punishment of your decision,
Because of this Holy items appearence the holy grail will decide on 7 masters to fight in the war, and in return those 7 will summon their servants.