Assassin's Blade

Assassin's Blade is a one handed dagger with good bonuses to Strength and Agility.

Intended for use by Horde assassins, these charcoal-black daggers are designed for striking directly into the hearts of foes. Even when held up to the light, they do not glint or produce any sort of reflection that will give their wielder away. Its powers only work in the hands of a Horde assassin.

Source
The dagger is a random drop from mobs in Shadowfang Keep.