Noble Phantasm

The armaments carried by Heroic Spirits, made from humanity’s illusions.
Though they generally come in the form of things like swords, spears, or bows, there are also those Heroic Spirits whose Noble Phantasms are auxiliary armaments like shields, rings, or crowns.
They are solidified “mysteries,” and injecting them with magical energy while calling their true name will unleash their abilities.

Fate/side material: Fate Encyclopedia

Special skills or weapons derived from famous events or anecdotal stories involving Heroic Spirits. The name noble phantasm comes from how they arise from the mesh of people’s illusions regarding the event of story. They are activated by infusing magical energy and calling their true name. There is a great variety of noble phantasms possessed by Servants. They can be physical weapons such as swords, spears, and bows. There are also rings, crowns, and other items which provide buffs or supporting abilities. Noble phantasms are categorized based on their effective range/use, such as anti-unit, anti-army, anti-fortress, anti-world, anti-magecraft, and barrier-type. Noble phantasms, being derived from famous events, are closely linked with a Servant’s true name. Therefore they are typically closely held secret during the Holy Grail War. However, in this Fourth Holy Grail War, noble phantasms are released without restraint, despite knowing that enemies are likely looking on from the shadows. Especially Rider is guilty of showing off his big moves.

Fate/Zero Animation Guide I: Glossary of the holy grail war I

These are special skills or weapons used as a Servant’s trump card.
They often comprise of the beloved weapons that Servants had when they were alive, or anecdotal stories about the Servant that is reproduced as a miracle.
All Servants can use the legends about them as weapons.
There are several classifications of Noble Phantasms. Anti-Unit Noble Phantasms are used against single targets in one-on-one situations, while Anti-Army Noble Phantasms are used against large groups of enemies. There is also another type, Anti-Fortress Noble Phantasms, which are effective at destroying buildings, fortresses, and even terrain.
There is also the rare EX classification… which is given to extraordinary Anti-World Noble Phantasms, but few Heroic Spirits have Noble Phantasms of this level.
Noble Phantasms are all incredibly powerful unique skills.
However, Noble Phantasms are a manifestation of the legends about a Servant, so if a Servant uses a Noble Phantasm their true identity will be revealed to the other Masters. Even in the Moon’s Holy Grail War where battles are mostly one-on-one, the leak of a Servant’s information can lead to defeat. The best Masters can be said to be the ones who can continue to win while keeping the true name and Noble Phantasms of their Servant secret.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA