"Deep mushroom" refers to the entire group of fungi that grows underground in caves and many parts of the dwarven Deep Roads. Collection can be a dangerous task, as the Deep Roads are often infested with darkspawn. Because of this, dwarven merchants often recruit "casteless" hirelings for the job, and pay them a meager percentage of what they earn selling the mushrooms to surfacers.
The most common varieties used in the herbalist's trade are the Blightcap, Ghoul's Mushroom, and Brimstone Mushroom, almost all of which tend to carry the darkspawn's corruption. While they cannot transmit the disease, this trait often makes them quite poisonous. Deep mushrooms should only be handled by experienced herbalists and should never be consumed without first being adequately cleaned and prepared. Careless consumption has been known to cause insanity, severe abdominal cramping, and even death.
—An excerpt from The Botanical Compendium, by Ines Arancia, botanist