Master

A designation given to an individual, usually a Magus, who has formed a contract with a Ghost Liner called a Servant as their Familiar. The Master provides the Servant with Prana and a sort of "passport" into the modern era in return for the services of said Servant.

Goals
Traditionally, a Master is an individual who has formed a contract with a Ghost Liner type Familiar, for the purpose of acquiring the Holy Grail of Fuyuki City. There are no actual restrictions on how the contract is used, when it is made, or how long it lasts, but the Servant inevitably has goals of its own.

Creation
In order become a Master, a person must successfully summon a Servant through use of a specific Formal Craft ritual. Upon completion, the Master must formalize a contract with the Summoned Servant by acknowledging their respective roles. The process creates a link between the two, allowing the Master to support the Servant with their Prana or an external source.

In every known case, with one exception, Masters only occur through the power of the Great Grail of Fuyuki City. The Grail both selects prospective Masters according to their need, and provides the majority of the energy needed to manifest and maintain the Servant. It also provides the Command Spells, granting limited control of the Servant to the Master.

The one exception to the rule is the case of Illyasviel von Einzbern, who successfully summoned Berserker without the aid of the Grail through the techniques of the Einzbern family. Her Command Spells were physically inscribed on her body, with an unknown power source. The Grail did recognize her as a legitimate Master, despite the abnormality, as she had the aptitude to become the vessel of the Lesser Grail.

Properties of a Master
Under ordinary circumstances, a Master is always a spellcaster, usually a Magus with a Magic Circuit, who is selected by the Grail for that purpose. It is possible for an ordinary person to become a Master, as in the case of Kuzuki Souichirou, as well as untrained individuals such as Uryuu Ryuunosuke, though this is rare.

A Master that is providing Prana to a Servant will "leak" magical energy in a volume that is easily detectable to most spellcasters. The leakage is distinctive, allowing a Magus to determine that the Master has a contract with a Servant.

An active Master will also possess a Command Spell as a visible proof of their contract.

A Master is always a Master, whether they currently have a Servant or not. A Master without a Servant can make a pact with an unclaimed Servant at will, though having too many Servants is hazardous to the health of a Master due to Prana consumption. This also means that a Master can claim a new Servant regardless of the length of time since his last Servant.

Variations
There are several variations on the Master and Servant system.

Catalyst
When a Servant is summoned, it normally matches the personality of the summoner. This is a form of catalyst, in which the Master and Servant share similarities and naturally gravitate to one another. However, it is possible to summon a specific individual by using an object linked with that individual in the summoning. The more closely the item and person were associated, the higher the probability of obtaining that Servant. This allows targeted summoning of especially strong heroes, but with the risk that the summoned Servant will be ideologically/morally incompatible with its Master, thus lowering their efficiency in Grail War. It is also possible, in rare cases, for the Servant to possess a catalyst associated with the summoning Master, allowing the process to work in reverse.

Non Spellcasting Master
A Servant without a Master can form a contract with a normal person without a Magic Circuit through the sharing of Vitality in the same manner as certain types of other Familiars. The Master must still find a way to provide the Servant with Prana, however.

Book of False Attendent
This is a Mystic Code that can allow someone to become a surrogate Master by passing a limited form of another Master's Command Seals into a specifically prepared receptacle. This device is not as efficient as a genuine Command Spell.

Berserker
It is possible to intentionally summon a Berserker class Servant through the addition of a few lines in the Aria of the summoning ritual. This robs the Servant of sanity, but also grants them superior melee combat abilities.

Shared Mastership
It is possible to modify the summoning ritual in order to share the burden of a Servant. The only known case of this allowed the summoner to place the burden of providing Prana to the Servant on his wife, while he retained the Command Spells.

Avenger
An anomalous class that first appeared in the third Holy Grail War, the Avenger is described as an Anti-hero. It is only known to have happened twice in the history of the Grail War, but this Servant, and an eighth Master, do sometimes appear during the Grail War.