The piercer is a monster from AD&D. My information comes from the 1996 printing of the Monstrous Manual, as well as from my own experience DMing the creatures.
Piercers resemble limestone stalactites found on cave roofs. They range in size anywhere from one foot to six feet long. Piercers are virtually indistinguishable from natural stalactites.
Piercers are actually a species of gastropod that, without their shells, resemble slugs with long tails. Piercers hatch from a hen sized egg, which the parent lays in clutches of six to eight in out of the way areas of the cavern. When they hatch, they appear to be slugs feeding on fungi. After several months, the young climb the cavern wall, secrete a chemical that hardens into the stalactite-like shape, and then attach themselves to the cavern roof. piercers live in groups of about 10 members. Aside from mating, piercers are not social creatures.
Piercers are carnivores who attack their prey by falling from the ceiling and impaling their foe with the sharp end of their shell. While piercers are nonintelligent, those living together in a colony are aware of each other, and often fall simultaneously, to kill prey, and feed on the kill. Piercers can hear noise and detect heat from 120 yards away. If a noise or heat source stays stationary for a long time, a piercer will edge toward it, and then attack.
A piercer only gets one opportunity to attack, after which it must slowly scale a wall and resume a position on the ceiling. After a piercer has fallen, it is slow and fairly easily slain. In fact, it's only defense is an acid secretion that causes mild harm if it comes in contact with flesh.
A piercer can go without food for months. It stores food in a second stomach that can preserve food for long periods of time. Piercers also store large supplies of water, extracted from their victims. Piercers can maintain this water supply for months
Piercers can pose quite a danger to any who venture into un-explored caverns. It is recommended that one armors him/herself heavily before doing so.