Servant

Servants are Heroic Spirits summoned by the Holy Grail. Once summoned, they can be classified as familiars as long as they are bound to a Master. Servants are legends from the past, present, or future. Because summoning even one heroic spirit is a miraculous occurrence, summoning seven in their pure forms would be beyond the Holy Grail’s power. Thus, to facilitate the process, Servants are summoned into one of seven vessels prepared beforehand. These vessels are classified based on their primary roles. Heroic Spirits can be summoned into any “class” that reflects the abilities they had in life. While they generally appear as they were in their prime, those with some kind of curse or strong attachment to their appearance can even be summoned in the form they had just before death.
Although too powerful to be controlled by humans, the prospect of having a wish granted by the Holy Grail serves as incentive to cooperate with a Master. A Master who is chosen to participate in the Holy Grail War possesses three Command Spells that, when invoked, forces the Servant to obey the Master’s orders. However, few Servants actually appear to have a wish, while the others are either content to serve their Master or have other goals entirely.
It is to be noted that, originally, the Servant system was designed to summon only the souls of “true” heroes – benevolent, honorable, and selfless protectors of humanity and justice. But since the corruption of the Holy Grail in the third Holy Grail War, evil spirits can be summoned. These entities can still be considered to be Heroic Spirits if they were widely worshiped, respected, or feared in life.

Summoning
There are several factors that determine which Heroic Spirit the Master will summon. Performing a summoning ritual to interfere with the Greater Grail causes it to borrow the information of the main body of the Greater Grail in the Throne of Heroes. The information, the soul of highest purity of the Heroic Spirit, is taken into the container of the Class and becomes a created copy of the Heroic Spirit, a Servant.
It is possible for the Grail to simply select a Servant for the Master by choosing a Heroic Spirit with good affinity with them, but they can also designate the Heroic Spirit during the summoning ritual by preparing a catalyst with ties to the Servant that they desire. If they choose a catalyst for multiple Heroic Spirits, like the gates of Troy, the symbol of the Trojan War, the Heroic Spirit among the candidates that has good affinity with the Master will be chosen.

While the rituals are generally specific, their main goal is to act as a precaution to create a bond with a perfect, reliable Servant. They really only require a summoning circle, even if unskilled, and a catalyst can bypass the need of chanting a spell. Doing so can create aberrations like Gilles de Rais being placed into a normally unsuitable class or King Arthur having a bad connection with her Master that does not allow her to receive prana through normal methods. The Grail of Fuyuki requires seven Servants, so it will pick from anyone appropriate until it has all seven in cases where more capable magi have not summoned Servants already.

Examples of catalysts include:

King Arthur: Avalon, the sheath of Excalibur.
Heracles: The Axe-sword, a slab of rock from a temple of Heracles.
Medusa: Mirror dug up from a temple in Eritrea, an item with ties to an old Earth goddess of Greece.
Medea: Literature from Colchis with ties to Medea.
Sasaki Kojiro : The mountain gate.
Cú Chulainn: Jewel on which is engraved an original rune inscribed by Cú Chulainn that had been crafted into earrings.
Alexander the Great: Shard of cloth from his mantle.
Gilgamesh: The first snake’s Shed Skin.

When summoned, Servants are imparted with knowledge of technology, culture, science, and terms of the era they were summoned into, as well as skill in languages. This is largely to prevent Servants from periods significantly before the era of a Grail War in question from suffering crippling culture shock which would otherwise prevent them from fulfilling their duties, and also enable them to blend in with that era’s society. It is not all-encompassing, so concepts may have to still be explained to them. For example, the idea of dating may not be understood until it is framed into the similar method of courting of the Servant’s time.
Without such knowledge, a Servant’s potential would be crippled. This knowledge does not just cover history but all aspects of “modern” life during the Grail War in question, to the point that Servants can find things that would have been unthinkable in their lifetimes to be mundane or even worthy of scorn. They are not granted knowledge on other Heroes through the Grail, but rather from knowledge gained within the Throne of Heroes.

Spiritual Body and Material Body
“Spiritual Body” is a word that describes an existence with spiritual properties. In other words, a shape is constructed without relying on components with physical properties. On the other hand, “Material Body” means something that is identical in structure as our flesh bodies. A Material Body, just like us, cannot walk through walls, and will hurt and become wounded when beaten with an iron baton. However, a Spiritual Body is capable of completely disregarding such physical interference. Attacking a Spiritual Body with a baton or sword (unless they are magical items) will only hit empty space.
When Servants materialize, they acquire Material Bodies in this world. They can also switch to Spiritual Bodies through their will. Spiritual Bodies have the advantages such as low mana consumption, and are difficult to be detected by enemies. One of the properties of a Spiritual Body is that it is unaffected by physical interference, but conversely it is difficult for a Spiritual Body to affect a Material Body. As a result, Servants materialize when engaging in combat.

Although both are Material Bodies, the composition of Servants and Humans are drastically different. Naturally, it is impossible to transplant a portion of a Servant’s Material Body to a human.

Servants can revert to spirit form when not in battle at will, to reduce the burden on their Masters. In this form they can pass through inorganic substances that aren’t dense with mana; they are also invisible to the naked eye (except for the Master they connected with) and most forms of scrying in this state, facilitating the concealment of their identity. They cannot be affected by normal interference when dematerialized, but need to materialize for battle. As Human Spirits they are able to consume the souls and minds of people and convert them into energy. Their basic abilities do not change from such nourishment, but they become tougher as their magical energy capacity increases.

Defeating a Servant
When a Heroic Spirit emerges into this world, he first acquires a Spiritual Core. The Heroic Spirit then materializes as the body envelopes this Spiritual Core. In order to defeat a Heroic Spirit, one must inflict damage to the Spiritual Core. The Spiritual Core gradually diminishes as a result of massive magical energy expenditure or sustaining bodily damage. Under these adverse conditions, the Heroic Spirit’s expenditure of magical energy will escalate, and should damaged be inflicted through powerful magical energy, curse, or Noble Phantasm, the Spiritual Core will be destroyed, and the Heroic Spirit will no long be able to remain materialized. The heart and head are connected directly to the Spiritual Core, thus they are the weaknesses of Heroic Spirits. Sustaining damage at these locations will significantly weaken the Spiritual Core.

The battle between Servants can be called a process of gradually shaving off the magical energy that forms their bodies. Attacks that consume a great amount of magical energy will inflict a massive amount of damage on the opponent, but it will also weaken yourself in the process. In order to inflict a great amount of damage with minimum expenditure of magical energy, it is necessary to collect intelligence regarding the enemy and assault his weakness. Their main strength is their inability to be harmed by regular weapons, meaning that even though there are numerous destructive modern weapons, Servants are among the most powerful.

Beings other than Servants that are able to combat Servants are few in modern times. Arcueid Brunestud would always generally win against a single Servant due to her overwhelming strength, but would likely fall if two were to fight her at once. The members of the Twenty-seven Dead Apostle Ancestors and Aoko Aozaki can fight directly. Humans utilizing ten Command Spells would also be able to damage Servants.

Against Dead Apostle Ancestors, because both are types of beings that live in mystery, such battles would come down to ability. Servants will generally hold a slight advantage, barring compatibility issues, and the Knight classes, Saber, Archer, and Lancer will all be able to have solid fights. While certain Anti-Castle-class Noble Phantasms are extremely effective against those who overwhelm by sheer size or alienness, there are those capable of surviving being hit by such attacks through pure endurance. At the same time, those same ones with extreme endurance would have a tough time with certain Anti-Personnel Noble Phantasms.

Servant

Fate/False Catalyst Gamble_Kuma ZFel