bag of D4s

As an action, you can spread a bag of D4s to cover a square area that is 5 feet on a side. Any creature that enters the area must succeed on a DC 15 Dexterity saving throw or stop moving this turn and take 1D4 piercing damage. Taking this damage reduces the creature's walking speed by 10 feet until the creature regains at least 4 hit point. A creature moving through the area at half speed doesn't need to make the save.