Phantom fungus

Phantom fungi were a breed of predatory, mobile fungus feared by those underground for their natural invisibility.

"Phantom fungus — you get them in old tunnels and caves. It's a little one though. Full grown, it would have been chewing off Zuzzara's hip, not biting her ankles."

- The dwarf Mumchance.

Description
Ordinarily, the only details indicating the presence of phantom fungi were the sounds of soft squelching footfalls accompanied by a strange smell of mold. If rendered visible, phantom fungi appeared as a brown or greenish brown fungal mass supported by four stumpy legs. A cluster of nodules rested atop its main body and its gaping maw was lined with rows of teeth.

Abilities
Phantom fungi were constantly and permanently under the effects of greater invisibility, allowing them to attack while invisible. Invisibility purge spells could not dispel this status, but they did become visible a minute after being slain. Phantom fungi were also adept at moving quietly.

Combat
Phantom fungi might wait for a meal to come to them but usually quietly hunted for prey, roaming the world until they came upon suitable targets. While not especially intelligent, they had certain instinctive strategies and were capable of determining easy from difficult prey. They would attack lone individuals almost anywhere, but preferred open spaces when tackling groups, moving around slightly between each bite from its many teeth to make itself harder to find.

Ecology
Phantom fungi were solitary subterranean creatures. There were Lowerdark variants of phantom fungi serving the druidic Circle of the Depths in the Underdark city of Fluvenilstra.

The plane of shadow was known to host shadow phantom fungi. Fiendish and half-fiend phantom fungi were also abundant in Shedaklah, the realm of the Demon Queen of Fungi Zuggtmoy. Phantom fungi, being fungal monsters, were among the favorite and most commonly summoned minions of Zuggtmoy, and among her faithful fungoid servants who reveled in the unnaturally warm dampness and actively sought to destroy oozes that emerged from Juiblex's portion of the realm.

Biology
Phantom fungi had low-light vision, their sensory organs being the nodule cluster atop their main body.