For the short description in the stats part, put:
Hurts to step on.
For the longer one in the description one, put:
A [caltrop] (jack rock, star nail) is a device made up of four (or more) sharp nails or spines arranged in such a manner that one of them always points upward from a stable base (for example, a tetrahedron or tetrapod). In the military context, caltrops serve to slow down the advance of horses, war elephants, wheeled vehicles and human troops.
^
Caltrops come in a wide range of sizes, from small caltrops only a few centimeters across used by ninja to deter humans in soft-soled shoes to massive steel (Czech hedgehogs) or concrete devices (dragon's teeth) designed to wedge into tank treads. Scattered around an encampment at night small, sharp caltrops can cause attackers stealthily treading on them to raise their own alarm.
^
Modern police and security forces often use spike strips against vehicles with inflated rubber tires, a device similar in principle to a patch of caltrops.