Monster Manual, p. 252
False Appearance. While a piercer remains motionless on te ceiling, it is indistinguishable from a normal stalactite. Spider Climb. The piercer can climb difficult surfaces, including upside down on the ceilings, without needing to make an ability check.
Drop. Melee Weapon Attack: +3 to hit, one creature directly underneath the piercer. Hit: 3 ( 1d6 ) piercing damage per 2 squares fallen, up to 21 ( 6d6 ). Miss: The piercer takes half the normal falling damage for the distance fallen.
The piercer hangs on the ceilings of caves and caverns in large groups. When a living creature passes by underneath, the piercers drop down and attempt to kill them.
Underdark