Piercers had two eyestalks that curled along their shells. Their stony shells made them indistinguishable from stalactites, unless one were to notice their eyestalks upon careful examination. The majority of their internal organs were inside the shell, but its mouth and its mollusk foot were at the exposed base of the shell.
Piercers could be found in a wide range of sizes, with the largest being about 6 feet (2 meters) in length and 1 foot in width. Such large specimens could weigh as much as 500 pounds.
Piercers slowly climbed to cave ceilings using their muscular "foot" in a manner similar to common snails. They could hang upside-down from the ceiling indefinitely by this "sucker foot", waiting for prey.
Piercers were highly sensitive to sound and heat. When they sensed motion below them, they would release themselves from the ceiling, plummeting to the ground. If they successfully impaled a victim, they would slowly consume it. If unsuccessful, the piercer was often killed by the fall, cracked and smeared upon the cavern floor. Some scholars claimed that a piercer could slither out of its shell while consuming its prey. In any case, it was highly vulnerable while feeding. After finishing its long meal, it would have to crawl on its long journey back to the ceiling again. If the movement of potential prey stopped for long periods of time, the piercer might slowly adjust its position over the prey rather than waiting for the prey to move under it. If one piercer managed to slay a victim, several other piercers would sometimes fall in an attempt to join in the feeding.
Piercers could be found in groups, or "colonies", of between three and eighteen animals and inhabited all manner of subterranean locations but particularly near cave entrances or other high-traffic areas. Rumors existed of much larger colonies of upwards of a hundred animals in deeper caverns.
Piercers were hermaphroditic, but they mated in pairs. They always crawled back to the cavern floor to mate, where they first performed elaborate mating "dances" before stretching out of their shells to consummate the process. They were highly vulnerable during this time. They laid clutches of about six to eight eggs in cracks in cavern walls or floors.
A piercer began its life as a pointy-tailed, slug-like creature emerging from an egg about the size of a chicken's. The young piercer used its radulae, its so-called "toothed tongue", to feed on fungi and grit and sand. It would use the latter minerals to begin composing its shell, after reaching an age of several months. The shell began as a tiny thin point and developed into a thick shell. When the piercer had reached a shell-length of about a foot, it began to develop its distinctive eyestalks, which it could move backwards alongside its shell.
A piercer could live for long periods of time without food, and alchemists believed that piercers had a second stomach, which secreted a special substance that could keep food fresh for longer periods of time than normal, allowing them to store it before digesting it fully. Piercers extracted the water that they needed from their victims.
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- Chris Elliot & Richard Edwards (April 1983). “The Ecology of the Piercer”. In Kim Mohan ed. Dragon #72 (TSR, Inc.), p. 13.
- David "Zeb" Cook et al. (1989). Monstrous Compendium Volume One. (TSR, Inc). ISBN 0-8803-8738-6.
- Doug Stewart (June 1993). Monstrous Manual. (TSR, Inc). ISBN 1-5607-6619-0.
- Gary Gygax (December 1977). Monster Manual, 1st edition. (TSR, Inc), p. 78. ISBN 0-935696-00-8.
- Chris Elliot & Richard Edwards (April 1983). “The Ecology of the Piercer”. In Kim Mohan ed. Dragon #72 (TSR, Inc.), pp. 12–13.
- Ari Marmell, Anthony Pryor, Robert J. Schwalb, Greg A. Vaughan (May 2007). Drow of the Underdark. (Wizards of the Coast), p. 157. ISBN 978-0-7869-4151-3.