I'll cut to the chase. To use this you need to be a Rogue and you need to be Horde (or at least I don't think there is an alliance equivalent to this quest).
1) Be a Horde Rogue
2) Do the quest chain Need to Know - Quest - World of Warcraft until you get to The Perfect Dissemblance - Quest - World of Warcraft.
3) Use the item on a raven priest
4) Stealth
You character will (or should) turn completely invisible. All that is left is the little round shadow on the ground. It's too bad that everyone on the server is a douche so I couldn't test out whether it's clientside (doubt it) or whether or not they can click on you.
I don't know what you could use this for. Mabye make a bet with someone concerning finding you, or cheat in a duel... you could sneak up on someone real easy while invisible.
My brother discovered this, but he doesn't use MMOwned so I'm just posting it for him. Sorry he didn't take screenshots of it so someone will just have to test it hmm?
Edit: Flame me, no proof, I know >_>