Caltrop

A caltrop is a metal ball bristling with metal spikes or prongs.

When a caltrop is left on the ground, there is always at least one spike standing more or less upright, ready to pierce the foot of the unwary. In order to be effective, at least 10 caltrops must be dropped in an area of 25 square feet (a 5ft x5ft square).

They are small, usually only 2in. in diameter,hard to spot, and most often dipped in a paralyzing poison that can disable a person quickly. If one has an average or below constitution, and is poisoned, they must receive healing as soon as possible, or end up flat on their back for a week.