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
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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