Phantom Blade

The Phantom Blade is a Level 44 quality One-Handed Sword with a Chance on hit to decrease a target's armor and prevent them from using stealth or turning invisible.

Source
This item is crafted with Blacksmithing (270); taught by Plans: Phantom Blade.

Background
During the last war, the Lich King outfitted some of his incorporeal undead warriors with special armor, making them difficult to deal with. In response, human craftsmen developed the phantom blade. These longswords are a translucent white color, and seem to disappear almost entirely in sunlight.