The Night Blade

The Night Blade is an epic dagger with an armor reduction proc.

Source
This item is a world drop. It also drops from Khorium Lockboxes.

Patch 2.1
As of Patch 2.1, the Night Blade has received a nerf and a buff. The DPS (damage per second) has increased to 81, but its proc has been reduced by a significant amount, making it still a balanced weapon.