Ghost Mushroom

Ghost Mushroom is an herb that can be gathered with Herbalism (245).

Source
As far as Azeroth goes, Ghost Mushrooms can be found only in Un'Goro Crater (in Fungal Rock). They grow exclusively underground. Each node yields only one Ghost Mushroom with a relatively high respawn time compared to other herbs.

In Outland, they have been spotted in caves in Zangarmarsh. The frequency of spawns seems to be dramatically higher in these caves than in Azeroth. A popular spot is the Funggor Cavern, in the south-eastern part of Zangarmarsh. Often up to three nodes are spotted at one time.

Nature elementals such as Irontree Stompers, Irontree Wanderers, Withered Protectors, and all the Warpwood mobs in Felwood will occasionally drop a Ghost Mushroom.

Until the Darkmoon Faire was revamped in Patch 4.3.0, Lhara would sell up to 4 Ghost Mushrooms (limited supply) for 15 silver each.

Pickable Zones

 * Un'Goro Crater
 * Maraudon
 * Zangarmarsh

Alchemy

 * Alchemist's Flask
 * Elixir of Demonslaying
 * Elixir of Shadow Power
 * Ghost Dye
 * Invisibility Potion
 * Limited Invulnerability Potion

Inscription

 * Violet Pigment
 * Ruby Pigment