Sword of Promised Victory.
Sword of light. It is not a man-made weapon, but rather a divine construct tempered by the planet. It is the Noble Phantasm standing at the very pinnacle of holy swords.
The strongest illusion (Last Phantasm), crystallized and refined from the wishes of humanity stored in the planet.
Though it was entrusted to elementals, who act as the planet’s sense of touch, it temporarily passed into the hands of a human king for a time.
Because it is so recognizable as the symbol of King Arthur, Saber had no choice but to keep it hidden while she fought.
But despite the fact that it’s a Noble Phantasm that’s almost synonymous with Saber herself, the number of times she actually uses it can be counted on two hands.
That is, three times in the Saber route, once in the Rin route, and twice in the Sakura route.
Because it is too powerful, it cannot be used carelessly at ground level.
Whether it be dark or light, it could be argued that Excalibur’s clash with Rider’s Bellerophon is Fate/stay night’s grandest splendor.

Fate/side material: Fate Encyclopedia