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.
The dagger is a random drop from mobs in Shadowfang Keep.
It is the Twink Dagger of choice for level 19 Rogues. Most common enchantments are +15 Agility and Crusader. A cheaper alternative for PVE is from a quest chain in Ghostlands that starts with The Farstrider Enclave (Horde Only).
Since patch 2.3 it has been suspected that it will only drop off of level 21 or higher mobs, and since the mob level nerf it will only drop off bosses in Shadowfang Keep.