Conjure Swarm of Rats

You create an inky portal on a flat surface you can see within range. A skittering swarm of rats erupts from the portal that lasts for the duration or until you cast this spell again. When you cast the spell, make a melee spell attack as the rats bite at a creature within 5 feet of them. On a hit, the target takes piercing damage equal to 2d4 + your spellcasting ability modifier. 

As a bonus action on your turn, you can will the swarm to move up to 30 feet and repeat the attack against a creature within 5 feet of it.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the rats' damage increases by 1d4 for every slot level above 2nd.