Obsidian Blade Necklace – "This is Obsidian–volcanic glass. The edge is wicked sharp and will cut through anything," he explained quickly. "It's the only thing on this planet, besides us, that can kill the Arum. This is their kryptonite." (The Lux series by Jennifer L. Armentrout)
The Nazgul. Called the Ulair in the tongue of the elves, these nine once bore Rings of Power. But they could not see the Enemy for what He was. Now they are His fingers, leeching foul darkness into Arda.