The lightsaber is an iconic symbol of Star Wars, but the Darksaber - featured in The Mandalorian - is unlike any other. The traditional tool of the Jedi and the Sith, a lightsaber is far more than just a weapon. Its core contains a kyber crystal, a living crystal that a Force-sensitive can use to bond with the Force. The color of a lightsaber's blade is decided by the kyber crystal contained within it; Sith wield crimson blades because they have bent the crystal to the dark side.
The Darksaber is Star Wars' only black lightsaber. It also has unique properties, because it is heavier to hold than the average lightsaber - and its weight appears to increase if the person holding it lacks in confidence. According to the Jedi Kanan Jarrus, this is because the thoughts and emotions of the wielder are drawn into the Darksaber's own energy, with inner turmoil adding weight. No other lightsaber behaves in this manner; non-Force-sensitives have been able to use conventional lightsabers with no difficulty (albeit with less precision than a Jedi or a Sith). All in all, then, it appears the Darksaber really is unique - and Star Wars lore offers a potential explanation as to why.
The Kyber Crystal Inside The Darksaber Must Be Black
Every lightsaber contains a kyber crystal, which functions as the source of its power - and makes a lightsaber far more than a mere weapon to a Jedi or a Sith, for it can be used to access and channel the Force itself. As noted, the kyber dictates the color of the lightsaber blade - and Star Wars has never shown a black kyber, meaning the crystal at the heart of the Darksaber must be different to those used by the rest of the Jedi. This is the key to explaining the Darksaber's unique nature.
The Jedi of the prequel trilogy had settled on obtaining their kyber from a single world, one they considered sacred; the planet Ilum in the Unknown Regions. This is why all their different lightsabers have similar properties - making the same kind of humming sounds, drawing on energy in the same ways, and capable of the same feats. No black kyber has ever been seen on Ilum in canon, which explains why none of the Jedi of the prequel era possess a weapon like the Darksaber. The kyber contained within the Darksaber must have come from elsewhere.
The Darksaber Must Contain Kyber From Another World
The Darksaber was created by the Mandalorian Jedi Tarre Viszla over a thousand years before the events of Star Wars: Episode I - The Phantom Menace. Presumably the Jedi of this era had not set themselves on using kyber crystals from Ilum, meaning there would have been a far wider variety of crystals in use. According to the Star Wars: Force & Destiny sourcebook Gadgets & Gear, different types of kyber crystal are found scattered across the galaxy, and they create lightsaber blades with remarkable properties. Rare Solari crystals generate unusually large blades, ideal for swatting aside blaster fire; Ghostfire crystals from the Outer Rim create translucent blades that make almost no sound at all. Presumably the kyber crystal found by Tarre Viszla is even more unusual, creating the Darksaber's distinctive appearance, and giving it the attributes seen most notably in The Book of Boba Fett, when Din Djarin found it increasingly difficult to hold the Darksaber.
Tarre Viszla may have found a rare type of kyber crystal, but it cannot be the only one of its kind. Given this is the case, it is possible Star Wars will indeed introduce more blades like the Darksaber. These are unlikely to be found in or after the prequel era, given the significance of Ilum to the Jedi of the prequel trilogy, but they could well turn up as Star Wars continues to expand into the High Republic Era.