The Ghostly Skull is sold by a vendor. He can be found in the Underbelly of Dalaran in the area where the Rogues go for training. I think it is called the Black Market.
Yes straight back in black market, it's 40 gold and soulbound. it floats and is semi-transparent and it follows you to the side with glowing face around the empty eyes and a glowing flapping mouth. I call it a "blind date" because it never shuts up, kinda hangs out while I ignore it.
Even better if you choose to put 10 dollars out you can get a mini version of Naxxramas' final boss. Lil'K.T. will laugh when you kill people in pvp and kills critters.
