Gáe Bolg

Gáe Bolg is the Noble Phantasm of the Servant Lancer. It is a cursed spear that was obtained in the Country of Shadows by the Heroic Spirit Cú Chulainn. Wounds inflicted by Gáe Bolg do not heal while the spear remains in the world.

Gáe Bolg functions as a weapon in two different ways, which vary to such a degree that they are treated as separate Noble Phantasms.

The Spear of Impaling Barbed Death
A surely fatal strike from the cursed lance, Gáe Bolg. This attack always strikes the heart. By unleashing the abilities of the spear, the wielder can reverse the nature of causality. In effect, the cause is that "the heart is pierced" and the effect is that "the spear is thrust". Thus, the lance has already hit the heart before it is thrust, and the actual action is merely a formality that occurs due to the piercing of the enemy's heart.

As a consequence, dodging or guarding is impossible; the lance will simply change course so that it pierces the victim's heart despite any interference. This is exemplified in Lancer's fight with Saber, in which Gáe Bolg changed direction at an impossible angle so as to aim for Saber's heart. Once initiated, the only defense against this attack is a high degree of luck, which may save a potential victim by altering their fate so that their heart is not pierced. In spite of this reverse-causality property and the attack's resulting accuracy, use of Gáe Bolg is economical, consuming so little prana it can be used seven times without pause for recharge.

This use of Gáe Bolg is said to be based on Odin's spear from Irish Ulster mythoology, (titled The Declaration of the Elder God). This is an anti-unit Noble Phantasm arranged by Lancer, but the original use of Gáe Bolg was as a thrown anti-army weapon, as detailed below.

The Spear of Striking Death Flight
This technique utilizes the full potential of Gáe Bolg's curse, converting prana into energy and releasing the accumulated power after the spear is hurled, detonating on impact with enough power to blow away a multitude of enemies. In contrast to the Impaling Barbed Death property, the Striking Death Flight attempts to blow away large groups of enemies, instead of dispatching a single foe with absolute accuracy.

This use of Gáe Bolg is said to be based on Brionac (titled The Five Roaring Stars), the lance of the Celt god of light Lugh, Cú Chulainn's father. It is the way that Gáe Bolg was intended to be wielded.

Note-
''It seems as though "Brionac" originated from the video game "Seiken Densetsu"; there is no real evidence in any texts to support that "Brionac" was a spear that Lugh actually owned in mythology. In reality, the spear that Lugh was said to have was called "Areadbhar" (titled "The Slaughterer") the poisoned spear stolen from Pisear, king of Persia.''

Sources: Fate/Stay Night game translation v1.0a by Mirror Moon, the Fate/Side Material encyclopedia entry (p.61) and the dictionary resource at.