Holy Grail

The Seihai (聖杯) is the Christian holy relic that received the blood of. There have been many claims of its existence, 726 of which were investigated by the Church, and many conflicts for such artifacts have happened around the world throughout history. The name is utilized in the various Holy Grail Wars as the name of an artifact purportedly capable granting any wish to whomever comes to claim it rather than the real artifact, though the Church often gets involved simply because of that reason. Its advent is the ultimate goal of the various Holy Grail Wars.

Fuyuki
The Holy Grail of the Fuyuki Holy Grail Wars of Fuyuki City appears at the end of the war to grant the final Master and Servant pair a wish on the Great Grail, the true device that has the power to grant wishes. Because it is only a vessel to summon the Great Grail, it is known as the Lesser Grail.

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 sticks to the legend of the name. It is a gate that leads to the Root, Akasha; a hole connected to the nuclear core, the Greater Grail (大聖杯), 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 bring forth one's own will.

It is, however only a clump of magical energy and thus is not as omnipotent as people believe it to be, so wishing for something like "Saving the World" without specifying a working method will not suffice. Even if someone lacks power or wealth, the Grail can possibly carry out such a wish if they knew "the method of Saving the World." Kiritsugu Emiya, knowing only the practice of "sacrificing few for the benefit of many", was shown such a vision of "what would happened if the World was saved", though the corruption caused him to be shown the most malicious imagery of his "saved world." Had he followed through with it, the Grail would have realized it with certainty. Ultimately, it can only amplify the scale of realization of wishes with working methodologies and is incapable of granting true miracles.

Corruption
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 element. 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.

The servant who is known as Angra Mainyu was summoned in the Third Grail War, he is a Heroic Spirit born into existence according to people’s wishes. As a result, Avenger was born into this world in order to grant everybody’s desire for an existence like “All the World’s Evil (Angra Mainyu)”

Even though the Holy Grail would happen every 60 years, the 10 year gap between the Fourth and Fifth Holy Grail Wars was shorter because the Lesser Grail was destroyed before the Greater Grail could direct the accumulated mana to establish its pathway, with the unused mana becoming trapped in the Grail. The function of the Greater Grail was also already disrupted when it was impregnated with Avenger in the Third War.

Additionally, since the corruption, the Grail has been able to summon evil-aligned Heroic Spirits; something that wasn't allowed before. Heroic Spirits are humans whose legends are passed down in the memory of humanity, regardless of their alignment; the Grail originally was allowed to summon only "good" Heroic Spirits, but ever since the corruption, spirits like Gilles de Rais, Medusa and Medea can be summoned despite being anti-heroes rather than heroes in the common understanding of the term.

The classes are, however, unaffected. The summoning of Avenger was a mistake done prior to the corruption, and the corruption itself had no known influence on the classes. It seems that it is only a coincidence that the two known Casters have an evil alignment.

This power also gives birth to Heaven's Fall, the Reverse Moon (天の逆月), which is not the Grail of Heaven (天の杯), but Angra Mainyu, the core Grail. While it is weak compared to the maelstrom of prana that is the Einzbern Grail, what it can do is grant wishes. In a way, it is a small but "true" Holy Grail. It is the foundation of the looping world in hollow ataraxia, where it appears as a black moon, hence the name.

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. The Holy Grail chooses it Master by selecting magi possessing magic circuits, be of sufficient stature to summon a Heroic Spirits. The Three Houses take priority as Master.

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. When the Holy Grail goes into hibernation, it puts its accumulated mana into storage. When 7 Servants are present, their spiritual cores will be released and the Heroic Spirits’ bodies will disappear from this world. The cores alone will then return to the Throne of Heroes. 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 on Mount Enzou, 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.

Container
Up until the third Grail War, the vessel for the Grail, also called the Lesser Grail (小聖杯), was a literal "cup". However, during the events of the third War, it was damaged in battle and the ceremony was interrupted; for that reason, it had to be made as something that could manage and protect itself.

During the Fourth War, Irisviel von Einzbern was a homunculus forged by the Einzbern family to be the physical embodiment of the Lesser Grail. Her deteriorating condition as her consciousness was supplanted by the Grail was mitigated by implanting Avalon into her body until the final days of the war. When she was murdered by Kotomine Kirei, the Grail took on her persona.

During the fifth War, the Lesser Grail was made as the heart of Illyasviel von Einzbern, a special homunculus made with concentrated alchemy, with Irisviel as her mother and Kiritsugu Emiya as her father. The Grail hidden in Ilya was even more refined than the one in Irisviel, proof that Ilya was expected to be the Einzbern's trump card.

At the end of the Fourth War, Zouken Matou acquired the fragments of the destroyed Grail. He synthesized it with his Crest worms, which he put inside the body of Sakura Matou. Though she was incomplete, this turned her into a second Lesser Grail.

Greater Grail
The Greater Grail (大聖杯) is the gateway linked to the Root and the ultimate prize for the Fuyuki Holy Grail War, and is in essence, a wishing machine. Known as the core world, the Cup of Heaven (天の杯), the core of the Grail was formed by Justeaze Lizrich von Einzbern, who continues to remain at its centre. The creation of the Great Grail was done so under the supervision of Kischur Zelretch Schweinorg, the teacher of the three founders of the war, Justica, Makiri Zouken, and Nagato Tohsaka.

The magic circle of the Greater Grail is inscribed upon the land managed by the Tohsaka family in Fuyuki City. Located in the great cave underground of the Ryuudou temple on Mount Enzou, it consists of a series of seals carved on a radius of above 50 meters of bare rock. It is the most ideal location for the Holy Grail to be summoned, though three other locations also exist should the location at Mount Enzou be unavailable. The Greater Grail itself is a system, and the Holy Grail is the key to start it up. The Greater Grail does not have a will and Justicia’s soul was completely sublimated when she sacrifice herself for the ritual. While Holy Grail collects the souls of the Servants, the Greater Grail uses the energy to open a gateway. Though the Lesser Grail is forcibly destroyed by Saber under the command of her master, Kiritsugu Emiya, its corrupted contents spill into the Greater Grail in the form of black mud.

For the Holy Grail Wars, the Greater Grail is what selects the Masters and invites Heroic Spirits, setting them in the Servant class system. After picking a Master from each of the three founding families (the Einzbern, Matou, and Tohsaka), it preferably picks Masters who earnestly want it, although it is possible to have irregulars, like Kirei Kotomine in the fourth Holy Grail War, who has no sense of purpose.

In the Fate and Unlimited Blade Works routes of Fate/stay night, ten years after the Fifth Holy Grail War, the Mages Association planned on bring back the Greater Grail with great opposition from the Church. After the end of a great turmoil with the same magnitude as the Grail War, the Holy Grail in Fuyuki City was dismantled under the direction of Lord El-Melloi II and Rin Tohsaka and the Grail Wars of Fuyuki came to a complete conclusion (this event is indeed canon). In the Heaven's Feel scenario, the Greater Grail is either finally destroyed by Shirou Emiya (normal end) or shut down for good by Illyasviel von Einzbern (true end).