Both weapons and magic can be used for good or evil. Most fantasy stories feature blades and spells on both sides of the battle, divided by ends rather than means. Some might argue that using incredible arcane power to enhance a tool designed for killing is evil, but some cursed weaponry goes above and beyond.
Fantasy stories are almost always morality plays. The average fantasy villain is a guy on a throne with immense power and armies of slavering minions while the average fantasy hero is a humble citizen rising against incredible odds. Power is almost always the real antagonist, but sometimes the hero is as tempted by that seductive force as the villain.
Evil weapons are tools of death, typically swords, which have been somehow cursed to actively seek bloodshed and encourage violence. Heroes often wield some sort of chosen weapon as they face the villain's hordes. The blade of evil's bane is almost always capable of banishing the malevolent forces that threaten the realm without having to butcher anyone the old-fashioned way. Evil weapons, on the other hand, are typically alive and hungry. Sometimes they've been cursed. Sometimes they've absorbed the murderous intent of their user or creator. Sometimes they're a living being that happens to take the form of a battle-ax or whatever. No matter how they became what they are, evil weapons exist to encourage their wielders to kill. They typically offer fantastic power, in exchange for the blood that they crave.
Sometime around 1500, Sengo Muramasa was born in Kuwana, Ise Province, Japan. Very little is known about his actual life, but his career and his legacy were dedicated entirely to the craft of making katana swords. Muramasa's swords were praised in their time and favored by the most powerful people in the era, the shogun Tokugawa Ieyasu and his vassals. As the shogunate claimed power, Tokugawa cracked down on private sword ownership. A fictional story emerged from within the government claiming that the head family of Japan believed Muramasa swords to be cursed. This led the items to be valued as artifacts of the resistance and spread the legend of Muramasa's blades as mystical tools of death. By the 1700s, many believed that Muramasa's swords were cursed. If they were ever drawn, they couldn't be sheathed until they drew blood. This widespread belief in a cursed blade that thirsts for blood have been hugely impactful to fiction across mediums.
If a sword called Muramasa makes its way into a fictional work, one can assume it'll be an unstoppable tool of death wielded by the most powerful of warriors. Marvel borrowed the name and the man himself to create the only weapon that can slay the immortal Wolverine. Metal Gear Rising: Revengence features Jetstream Sam's brilliant red katana, the Murasama, which borrows half of the man's name. Like any other moderately well-known historical figure, Muramasa has a presence in the FATE franchise. There's even a 1964 episode of The Twilight Zone that focuses on a haunted blade strongly implied to be forged by the 16th-century master. Just about any cursed samurai sword owes a debt to the legend of the Muramasa, but other fictional blades thirst for death.
The Norse love a sword that demands a kill every time it's drawn. The best known is Tyrfing, which was first wielded by Odin's grandson but passed from one warrior to another over generations. Players of Final Fantasy XIV or the Castlevania franchise might remember adding the blade to their arsenal. There's also Dainsleif, which lends its name to a character in Genshin Impact and can be wielded in Castlevania as well. These blades, along with Muramasa's legacy, make up the history of the evil weapon trope across both the Eastern and Western canons. Tons of fictional works just use the weapons as they exist in the mythos, but many simply change the names, while changing very little.
Evil weapons are common enough to feel generic, despite their strong basis in myth and folklore. The curse that makes a weapon powerful can just as easily make it a danger to the user. A hero always has to struggle against the villain, but the ability to resist the desire for power is what makes them heroic. Plenty of fantasy stories see a protagonist make use of incredible power to take down their foe, but the evil weapon offers the ability to get carried away. From video game moral choice systems to powerful tools in film. From items in tabletop RPGs to comic book MacGuffin. The evil weapon is a simple trope that can fit into just about any story. Magic swords are key to fantasy, and adding a grim edge to that long tradition of mythical weaponry is a fun evolution. Some tropes have evolved over generations, while others feel ripped straight from the mythology that inspired modern stories.