“Elric knew that everything that existed had its opposite. In danger he might find peace. And yet, of course, in peace there was danger. Being an imperfect creature in an imperfect world he would always know paradox. And that was why in paradox there was always a kind of truth. That was why philosophers and soothsayers flourished. In a perfect world there would be no place for them. In an imperfect world the mysteries were always without solution and that was why there was always a great choice of solutions.”
― The Elric Saga Part IWhat makes the Black Swords so damn dangerous?! According to the Stormbringer fandom wiki entry on Stormbringer's history; "Before the Melnibonéans' ancestors, the Mernii, arrived on Elric's World, the Nihrain were a prosperous society who served the Cosmic Balance. To combat the Dead Gods and other powers of Law and Chaos that threatened to tip the scales of the Balance, the Nihrain forged the pieces of the Dragon Sword into the twin Black Swords, Stormbringer and Mournblade, and perhaps also forged their many brothers as well. Out of fear for the blades' power to slay them, the Dead Gods killed themselves (removed their physical forms and joined Eternity). But while both blades had the power to absorb vitality as did all Black Swords, Stormbringer was different from the others, because a mysterious power entered the blade at the time it was forged"
And one of the most dangerous magical quality that both Stormbringer & Mournblade have is the fact that they can slay divinities & gods; "Godslaying- Stormbringer is fully capable of killing gods, even Chaos Lords and Lords of Law, and is greatly feared because of this. Though it is said that this does not truly destroy the Lords of the Higher Worlds, it just consumes their current manifestations in that world and ensures they cannot return there."