An Immortal Weapon
Stormbringer is a magic sword from the Elric of Melniboné series of novels by Michael Moorcock. Most notably among magic swords, it has a completely black blade. Functionally, it acts as an extension of Joseph Campbell's monomyth, which explores the idea that the hero of every work of legend is the same character reincarnated, facing the same challenges, with slightly tweaked parameters. Stormbringer reimagines the monomyth as the hero's weapon driving the stories of the ages. King Arthur was a regular guy, but Excalibur, the sword which defined him as king, was actually an intelligent and immortal weapon that chose him to be the hero and guided him along his journey. The paladin Roland bore the blade Durandal, and the man remembered as Thor once carried Mjolnir. Each of these legendary weapons was the weapon Stormbringer carried by different men throughout history.
A Force of Evil
The twist behind the entire charade is that Stormbringer is unredeemably evil. The blade seeks to promote chaos and entropy, and is gifted with extraordinary power to do so: it can slice through most any material and can drink the very souls from its victims. However, at the same time it drains its user, leaving them powerless without the strength and vitality it provides them, and manipulates them into slaughtering innocents to feed its bloodlust.
No matter what iteration of the blade, its user is as much as slave as it is a wielder. Stormbringer constantly whispers in its bearer's ear, nudging them toward violence and betrayal. In spite of this, perhaps by Stormbringer's subtle influence, the user is almost always considered a hero when its violent acts lay low mighty beast or an entire army. In this way, the monomyth grows and Stormbringer (by whatever name it chooses) is recorded into the history books again, misremembered as a hero's weapon.
A D&D Classic
Naturally, this fits brilliantly into any campaign setting. It's an immortal intelligent weapon that can exist in every campaign setting, and in every legend. And D&D even has a canon version! And it's in the DMG!
Blackrazor is an artifact weapon introduced in the classic module White Plume Mountain, where the players are tasked to retrieve three weapons of legend from a dungeon, a trident, a warhammer, and a black-bladed sword. It's easily the most memorable out of the three, and easily the most dangerous.
As my players surly know, I want the characters in my campaign to become mighty titans by story's end, but I don't give away power easily. Stormbringer is my go-to gambit of power, a manipulative puppet-master of a weapon that believes in entropy and drives the player to kill. The careful player can turn it into an immense boon, and the reckless one can turn their character into an enemy within the ranks.
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