Piercers are a monster class represented by p. They hide in the ceiling, surprising your character by falling on you just as you pass beneath. The fall itself can deal 4d6 damage, but wearing any metallic helmet will fully protect you, the blow glancing off it. After falling or otherwise being discovered, piercers may still attack but are extremely slow.
In order of increasing toughness, there are rock piercers, iron piercers, and glass piercers.
Heaven piercer (dNetHack)
A somewhat higher-damage piercer found in Gehennom.
Yet another Dungeons and Dragons monster.
A piercer resembles a stalactite, but with eyes in the sides. It is actually a giant gastropod; the "stalactite" is its shell.
They cling to the ceiling, pretending to be a stalactite. When prey comes beneath them, they fall to impale it.
The adjectives "rock", "iron", and "glass" describe the materials that the respective piercers resemble, not what they (or their shells) are composed of. They leave ordinary meaty corpses when killed, and iron piercers do not rust (although glass piercers are acid-resistant).
Ye Piercer doth look like unto a stalactyte, and hangeth
from the roofs of caves and caverns. Unto the height of a
man, and thicker than a man's thigh do they grow, and in
groups do they hang. If a creature doth pass beneath them,
they will by its heat and noise perceive it, and fall upon
it to kill and devour it, though in any other way they move
but exceeding slow.