Holy Grail

The Holy Grail (聖杯).

Nature and power
What is handed down in the city of Fuyuki isn't the one that received the blood of the Christ, but the prototype of an even more ancient "omnipotent container"; this container is the 726th Grail that follows the legend of granting wishes. It is a gate that leads to the Root, Akasha; a hole connected to the nuclear core, the Great Grail, a wish machine - the key to startup the Great Grail. It is not an omnipotent container, but a replica created to work like a "wish machine" that can only bring forth one's own answers to his own questions.

The essence of its content is a "colorless force", but after the third Grail War, it has been polluted by Angra Mainyu and became a malignant "swirling power" - a curse, the third substance. Hence, it is absolutely impossible to correct inconsistencies by precise calculations or interactions, and it will interpret any wish of the owner with nothing else than destruction, and grant it. Moreover, once it has been opened, it will keep flowing out infinitely and bring disaster.

Usage
As a spiritual existence, the Kotomine Church manages the replica that will become its vessel, but the vessel of this fifth war is the heart of the homunculus Illyasviel von Einzbern. It is a catalyst that will help the descent of the real thing, and the ceremony to bring forth the cup that grants any wish, the Holy Grail War (or Heaven's Feel) requires seven Magi and Servants.

Servants that get killed return to being a "piece of energy" and disappear from the time axis, but the Grail (container) created by the Einzberns is here to temporarily stop that and store them. Once the vessel is filled with energy, the Great Grail can start the ritual that will open a "hole" to the Akasha.

To open a way to the swirl of the Akasha, the Grail needs to sacrifice all seven Servants to start up the Great Grail, but six Servants are enough if the wish is within the limits of the world.

There are four gates in Fuyuki where the Grail can descend, and a miss will have it switch to the next gate. The first time was at the Ryuudou temple, the second time was the Tohsaka mansion, the third was the church on the hill, and the fourth was the field that is now burnt.