Cantrops

You create 20 caltrops which cover a 5 foot area you choose within range. Any creature in that space when you cast the spell must succeed on a DC 15 Dexterity saving throw 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 must also make the saving throw when it enters or exits the area on a turn.

A creature moving through the area at half speed doesn't need to make the save.