One of my favourite dungeon creatures in previous editions was the piercer, a perfectly fit creature that existed in caverns. In third edition, it was nixed or morphed into something else depending upon how you view it. I liked the stupid simplicity of the piercer; you knew what it was, where it lived and what it did.
Early in my dungeon and monster creation days, I scoured almost every book I had at home, looking for inspiration. I noticed a wickedly similar creature in the old Star Wars Marvel comics, except they lived outside, and rather then falling down, they “jumped” up out of the ground. This seemed like a much better version of what the piercer should have been...so here is my melding of old, not-so-old and just a cool idea to use as a living hazard. It's done in Pathfinder but easy enough to convert.
A stalagmite thrusts upwards, tearing your ally apart, all around you dozens of similar stalagmites jump out of the ground, impaling your traveling party.
Nyssian Piercer CR 3
N Medium aberration
Init -2; Senses Tremorsense 60 ft., Perception +13
AC 17, touch 13, flat-footed 17 (+4 Natural Shielding, +3 natural)
hp 19 (3d8+6)
Fort +3, Ref +0, Will +3
Speed 10 ft.
Melee bite +4 d6 + 2d6 Surprise attack *
Str 8, Dex 9, Con 14, Int 5, Wis 10, Cha 9
Base Atk +1; CMB +4; CMD 13
Feats Improved Initiative
Skills Stealth +19*, Perception +13
SQ Tremorsense, Damage Reduction (5/-)
The piercer appears as a stone stalagmite, the smallest being around four-feet tall. This is its outer shell. Inside, the piercer appears as a slug-like creature with a long tail. Two tiny eye-stalks protrude from its sides.
The piercer attacks by thrusting upwards on unsuspecting individuals and impaling them with its shell. While one piercer is not too threatening, they usually exist in colonies, anywhere from a dozen to one hundred piercers stretching out covering a range of over a thousand yard radius. Piercers often attack creatures when the meal is in the middle of their colony, presenting a hazard for creatures unable to fly or otherwise escape from the middle of the colony.
They are not too bright, but their tactics are effective when they have surprise. They exist in any environment that they can tunnel into. When their attacks no longer bring sufficient food, they begin their slow journey to richer pastures, they have hearty constitutions so they can survive for months without proper nutrition. Many evil creatures deliberately lure them to act as living hazards to protect locations. If you know where they are, you can can easily avoid them while walking close to their lunging spot. They generally don't care for intruders as long as the colony has enough to eat.
Surprise Attack: If their victims are not aware of the piercers, on the surprise round they inflict damage as a surprise, same as a rogue of their HD.
Dmg Reduction: When piercers are below ground, they receive a 5/- dmg reduction as they are fully shielded. Out of the ground, they lose this protection.
Tremorsense: Piercers can sense any creature larger than 20 pounds within a 30 ft radius if they are moving while touching the ground.
Skills: The piercer receives a +8 racial bonus to Perception checks. The piercer receives a +15 racial bonus to Stealth checks when below the surface .