Excalibur

Excalibur, the sword of light, is the symbol of the king Arthur, and the Noble Phantasm of Saber. It is not a weapon made by man, but a divine armament forged by the planet. It sits at the top of the category of holy swords. It is a Last Phantasm, the ultimate fantasy, crystallized inside the planet from the collective wishes on mankind, as the crystallization of the prayer named glory. It was under the control of the spirits of the planet, but was temporarily passed down to the king of men.

It is a holy sword that converts the prana of the owner into light, intensifying the kinetic energy by convergence and acceleration, allowing the use of a sorcery at the level of divine spirits. A weapon of directive energy.

To a third party viewer, it looks like a huge band of light, though in practice, only the tip is what can hit, and the light is a result of the "dislocation" of everything that this "ultimate killing technique" goes through. The tip of the attack also holds heat, making it a wave of light that can mow through the surface of the Earth. The concentric area of impact is broad enough to incinerate an entire military cohort, as it ranks as an anti-fortress Noble Phantasm.

To release this ultimate secret technique that holds an energy equaling the total of Saber's entire prana, both hands are needed.

Black Excalibur
Excalibur is also the Noble Phantasm of Black Saber, as the sword of the black northern light - sometime nicknamed Black Calibur in English-speaking fandom. As it is an amplifier of the prana of the owner, the holy sword of light of the blackened Saber also becomes black. Just as Morrigan and Vivane, the fairies of the Lake, can coexist, this holy sword can also bear both evil and good.

Source: the dictionary resource at Jonnobi.