Ah, I thought of one.
The Subtle Knife, from the His Dark Materials trillogy, and the title of the 2nd book.
http://www.bridgetothestars.net/wiki/index.php?title=The_Subtle_KnifeThe Subtle-Knife, also known as
Æsahættr, meaning "
God-Destroyer", is the name given to the dagger created by the Guild of Torre degli Angeli. An ordinary looking dagger, it possesses extraordinary capabilities; able to cut through any substance - lead, flesh, Dust; even the membrane that separates one world from another.
"It was an ordinary-looking dagger, with a double-sided blade of dull metal about eight inches long, a short crosspiece of the same metal, and a handle of rosewood. As he looked at it more closely, he saw that the rosewood was inlaid with golden wires, forming a design he didn't recognize till he turned the knife around and saw an angel, with wings folded. On the other side was a different angel, with wings upraised. The wires stood out a little from the surface, giving a firm grip, and as he picked it up he felt that it was light in his hand and strong and beautifully balanced, and that the blade was not dull after all. In fact, a swirl of cloudy colors seemed to live just under the surface of the metal: bruise purples, sea blues, earth browns, cloud grays, the deep green under heavy-foliaged trees, the clustering shades at the mount of a tomb as evening falls over a deserted graveyard...."
The two edges of the knife are very different. One is clear bright steel that is incredibly sharp and will cut through any material in the world. The other side is just as sharp but silvery. This is the side that cuts into different worlds.
The knife is carried in a leather sheath, backed with stiff horn. There are buckles to hold the knife firmly in place because any movement would cut through the leather.
Bonus: Those chosen by the knife to wield it, once they grasp the handle, their ring and little finger fall off.
Major Series spoiler if anyone cares
The Wielder literally stabs God and kills him.
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unrelated
Obviously I'm an idiot and completely forgot what is probably one of the most important weapons in a fantasy series: Stormbringer.
All other magic blades are but shadows of Stormbringer, cast through dimensions.
Also, Saberhagen's 12 Swords of Power.
I just spent quite some time reading about the 12 Swords of Power... Do you recommend the series Curethan?
Same thing happened to me. And same question.