Swarm

Choose a target within range; a cloud of minuscule poisonous spiders floats on gossamer strands towards it. The target must succeed on a Dexterity saving throw to avoid the spiders or become poisoned until the end of its next turn.

This spell deals poison damage on a failed saving throw when you reach certain levels. When you reach 5th level, a creature takes 1d10 poison damage; at 11th level this increases to 2d10; and at 17th level this increases to 3d10.