Monster Module p.11
Antimagic Susceptibility. The caltrops are incapacitated while in the area of an antimagic field. If targeted by dispel magic, the caltrops must succeed on a Constitution saving throw against the caster's spell save DC or fall unconscious for 1 minute. False Appearance. While the shield remains motionless, it is indistinguishable from a normal shield. Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny caltrop. The swarm can't regain hit points or gain temporary hit points.
Pierces. Melee Weapon Attack: +4 to hit, one target. Hit: 14 (4d6) [roll4d6] piercing damage or 7 (2d6) 2d6 if the swarm is at half its hit point or fewer. Additionally, the target's walking speed is reduces by 10 feed until the end of their next turn.