Arturia Pendragon (Lily)

Overview

Short version: Fate frequently uses historical fact and real world mythological canon as more of a loose guideline than a rule book. There are a lot of aspects of Fate that muddle or flat out contradict Arthurian legends, as well as a lot of contradictions and poor or missing translations within Fate itself. I'll be noting and explaining the differences where I can.

Caliburn

In Fate, Caliburn is the sword that Artoria, at age 15, drew from the stone to prove herself the one true king of Britain. Upon being drawn, Artoria's aging was halted. It was her primary weapon before it shattered in combat as a result of Artoria breaking the rules of chivalry.

For it's mythological origins, read below.

Excalibur

In Fate, Excalibur was given the Artoria by the Lady of the Lake after Caliburn broke. It also came with a sheath called Avalon, which served to make Artoria effectively immortal due to it's remarkable healing capabilities.

In real world mythology, Excalibur's role is much the same. It's worth noting that Excalibur and Caliburn are the same sword - and, indeed, their names are simply variant spellings of one another due to a lack of standardized phonetic rules when the stories were being translated. However, while the story varies between different retelling of the Arthurian legends, Excalibur is usually not the sword that is drawn from the stone. Which brings us to...

Clarent

In Fate, Clarent is a third, unrelated sword that 'symbolizes royalty' and was likely meant to be passed to Artoria's heir. It was stored in Camelot's armory until it stolen by Mordred.

In mythology, Clarent is... a mystery. The few times it's mentioned it's described as a delicate sword meant for ceremonial purposes. It's wielded by both Mordred and Arthur at various points. Though there's no consensus on it's true identity, it's assumed by many to be the original sword in the stone.