Caladbolg (meaning "hard cleft" or "hard blade"), is an enchanted two-handed sword made by the fae of the Seelie Court. Forged from magical ore from the depths of Faërie, it all but hums with magical power, so much so that when swung it leaves the after-image like the arc of a rainbow. Unbreakable and keen enough to slice through solid stone, it was said to make the wielder powerful enough to defeat an entire army.
In practical terms, it is a supernatural-level weapon, however it cannot be wielded except by the will of the Seelie Queen, Lady Gloriana, who is its keeper. It is traditionally offered up for the use of a Champion, who may only wield it when the need is great, and only then in defense of innocent lives or defense of the Seelie Court. Angel has somewhat more discretion in his use of the blade, as he was a pre-existing Champion in his own right, but even he must be acting in the direct interest of the Powers for additional usage. Typically, this translates into getting 'direct orders' through one of Cordelia's Visions. (If the Champion Signature ability is active, the sword is operating at full power.) Further discretion is granted him to use the sword as Enhanced level weapon for personal use. When this happens, the sword does not leave its fabled rainbow arc, but it is still unbreakable.
The blade's exact form has evolved over the centuries, though it seems more that it has mystically evolved with the times, rather than being actively reforged. What remains constant about it is that it contains no iron nor steel, that the metal carries a faint rainbow sheen, and that it is always a two-handed weapon with a hearty heft to it.