Orb of Deception

Orb of Deception is a trinket used to change the model of the player's avatar into a model of one of the races belonging to the other faction.

Source
This item is a world drop, but also drops from all high end pre-Burning Crusade instances such as Blackwing Lair, Molten Core, Stratholme, and Scholomance. In both cases, the drop chance is extremely low (no more than 1%-2%).