Holy Grail

The cup that received the blood of God.
It is one of the greatest holy relics, a wish-granter that fulfills its owner's desires.
It is believed to have its origins in the “wish-granting cauldron” that appears in numerous myths.
The Holy Grail that appears in Fate/stay night is not the original, but rather a replica made to function as a “wish-granter.”
When Heroic Spirits are defeated, they return to their original forms as fragments of power and vanish from the time axis.
However, before that can happen the Einzbern Holy Grail intercepts and stores them until the ritual to open the “hole” can be enacted by the Great Holy Grail.

Fate/side material: Fate Encyclopedia