The Sword of Light
The Claíomh Solais, or the Sword of Light, is a powerful weapon once wielded by Nuada, the first king of Ireland. The sword is also one of the Four Treasures of the Tuatha Dé Danann. Throughout history, Claíomh Solais has taken the form of a variety of swords. And it has been wielded by a variety of heroes.
As a conceptual weapon rather than an actual weapon, Claíomh Solais is an abstract idea rather than a concrete entity. As a result, Claíomh Solais is not limited to a single form or even to a single wielder. Instead, it may take the form of any sword that it choses to possess and it may be wielded by any hero that it allows to do so.
The sword of light is a fixture of an Irish tale group describable as a quasi-bridal-quest. This characterization is inspired by the formula where the hero gains a beautiful wife (and riches) by gambling against a gruagach aka wizard-champion, but suffers losses which makes him beholden to mount on a hopeless-seeming quest. Like the actual "giant's daughter" bridal quest tales, the sword of light hero often gains assistance of "helpful animals" in completing his tasks or ordeals.