Caltrops

As an action, spread a bag of caltrops to
cover a 5 feet square. Any creature that
enters the area must succeed on a DC 15
Dex save or stop moving this turn and take
1 piercing damage. Taking this damage reduces the creature's walking speed by 10
feet until the creature regains at least 1 hit point. A creature moving through the area
at half speed doesn't need to make the save.