The Light Sword is one of the peculiar, strong, and fascinating weapons in One Piece, mostly associated with an Admiral in the Marine Patrol Forces, Borsalino, better known by his nickname Kizaru. This amazing sword is a result of Kizaru's Devil Fruit powers with the Pika Pika no Mi, which enables him to create, manipulate, and turn into light at his will. The Light Sword is not a physical, traditional weapon at all; it is something developed with pure light energy, which explains further the main feature of Logia-type Devil Fruits and their mighty power in the universe of One Piece.
Kizaru's Light Sword is called "Ama no Murakumo," which means "Sword of the Gathering Clouds of Heaven." His technique is one whereby Borsalino transforms a long, sharp, and durable sword from the light and cements it. What really shows one's advanced level of control over the power of the Devil Fruit is his being able to have a solid weapon manifested from light powers. The appearance of this sword is unique, often drawn as just a gleaming blade made purely out of light to further impress its otherworldly nature.
There are a few sides to the battle efficiency of the Light Sword. It's got the kind of power to cut like an ordinary sword, but it had the properties of the light which could make it faster and more unpredictable compared to conventional weapons. In such ways, the combination of physical sharpness and light-based properties makes the Light Sword of Kizaru in the hands quite formidable, well-graduated in both close- and far-range combats.
One of its most impressive outings would be in the Kizaru VS Silvers Rayleigh battle at Sabaody Archipelago. In this severe battle, Kizaru showed a technique that would lead to sword fighting: Ama no Murakumo, against Rayleigh, who happened to be a great swordsman himself. Of course, this encounter was meant to further drive home the Light Sword's potency and durability in resisting traditional methods of swordsmanship.
That made Kizaru's Light Sword design really interesting for fans, who also pointed out how it looked very different compared to a regular One Piece sword. It doesn't really look like a katana or any other relevant sword shape presented in the series, being of course really abstract since it's a manifestation of his Devil Fruit powers. There's a uniqueness to the design that seems to enhance the visual spectacle of Kizaru's battles.
In a larger perspective regarding the power systems of One Piece, Light Sword will find itself on the crossroads of Devil Fruit abilities and traditional weaponry. Now, granted, most of the swordsmen in the series use physical blades, often enhanced by Haki or other powers, but Kizaru is a clear display of how Devil Fruit powers can be used to forge completely new kinds of weaponry. In doing so, it opened up all the possibilities for combat within the world of One Piece, and it expressed just how creative and varied fighting styles could be within the series.
Due to its popularity, light-based weapons from One Piece have inspired different merchandise or collectibles. While none are exactly Kizaru's Light Sword, several light-up sword replicas are available for fans as often counterpart items based off other famous swords from the series. For instance, custom replicas of Roronoa Zoro's swords—Wado Ichimonji, Shusui, and Enma—that are made to light up are allowable so that fans can be freed up in terms of having the visual spectacle of energy-infused blades. These collectibles stand as testament to just how long light-based weapons remain captivating in the series One Piece.
Worth noting, in this regard, although Kizaru's Light Sword is one of the most prominent examples of light-based weaponry in all of One Piece, energy weapons themselves are not unique to him. Other characters with light- or energy-based powers have presented quite similar capabilities for creating weapon constructs. Though none reach even half the recognition that Kizaru's Ama no Murakumo gets.