Stormbringer is a magic sword featured in a number of fantasy stories by the author Michael Moorcock. It is described as a huge, black sword covered with strange runes, created by the forces of Chaos. The sword has a will of its own and it is hinted that the sword may be controlled by an inhabiting entity. It is wielded by the doomed albino emperor Elric of Melniboné. Stormbringer makes its first appearance in the 1961 novella The Dreaming City. In the four novellas collected in the 1965 book Stormbringer, the sword's true nature is revealed.

This powerful enchanted black blade is a member of a demon race that takes on the form of a sword, and as such is an agent of Chaos. Stormbringer's edge is capable of cutting through virtually any material not protected by potent sorcery, and it can drink the soul from (and thereby kill) any unprotected living creature upon delivering any wound, even a scratch. Its most distinctive features are that it has a mind and will of its own, and that it feeds upon the souls of those it kills. Elric loathes the sword but is almost helpless without the strength and vitality it confers on him.

Stormbringer's hunger for souls is such that it frequently betrays Elric by creating a bloodlust in his mind, turning in his hands and killing friends and lovers. The cursed nature of the sword adds to Elric's guilt and self-loathing, even as he feels pleasure when the stolen lifeforce enters his body.

Stormbringer has a "brother" sword named Mournblade, which was at one time wielded by Elric's cousin and enemy Yyrkoon. It is identical to Stormbringer in most regards. Later stories reveal that there are thousands of identical demons, all taking the form of swords. Three such sibling blades appear in The Revenge of the Rose and many more "brother blades" are seen in the novel Stormbringer, but only Mournblade and Stormbringer are named.

In Elric of Melniboné, Elric and cousin Yyrkoon find the runeblades in a realm of Limbo and commence battle. Elric and Stormbringer disarm Yyrkoon, and Mournblade disappears. Yyrkoon is defeated, and Elric and his cousin return to Imrryr.

In The Weird of the White Wolf, Elric returns to Imrryr after a long journey and confronts Yyrkoon, who usurped the throne in his absence. Yyrkoon has regained Mournblade through unknown means and uses it to attack. Elric and Stormbringer kill Yyrkoon, and no further mention is made of Mournblade until it is later disclosed that it was recovered by the Seers of Nihrain, to be wielded by Elric's cousin, Dyvim Slorm. Imrryr is sacked, though the pillagers' fate is not much better, being pursued by the golden battle barges and the few dragons who were awakened, led by Dyvim Tvar. Only Elric's ship escapes, propelled by the aid of his sorcery.

In Stormbringer, Elric learns that the representatives of Fate, which serve neither Chaos nor Law, recovered Mournblade from the netherworld. They present it to Elric and explain that the runeblades were designed to be wielded by those with Melnibonéan royal blood as a check against the might of powerful beings including the Dead Gods of Chaos. Elric gives Mournblade to his kinsman, Dyvim Slorm, and the two men become embroiled in a confrontation between the gods. Elric summons others of Stormbringer's demonic race (also in the form of swords) to fight against a number of Dukes of Hell, brought to the Young Kingdoms by Jagreen Lern, theocrat of Pan Tang.

Ultimately, Elric's reliance on Stormbringer proves his undoing: after the utter destruction of the Young Kingdoms in the battle of Law and Chaos, just as it seems that the cosmic Balance has been restored, Stormbringer kills Elric, transforms into a humanoid demon, and leaps laughing into the sky, to corrupt the newly-remade world once more. The sword-spirit says to the dead Elric: "Farewell, friend. I was a thousand times more evil than thou!"