This spell creates a swarm of tiny undead. Choose a pile of bones or the corpses of no less than a dozen tiny beasts within range. Your spell imbues the targets with a foul mimicry of life, raising them as an undead swarm. Use the statistics of either the swarm of rats or the swarm of ravens (at the DM's discretion), which are undead instead of beasts. 

On each of your turns, you can use a bonus action to mentally command the swarm you made with this spell if it is within 60 feet of you (if you control multiple swarms, you can command any or all of them at the same time, issuing the same command to each one). You decide what action the swarm will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor. If you issue no commands, the swarm only defends itself against hostile creatures. Once given an order, the swarm continues to follow it until its task is complete.

The swarm is under your control for 24 hours, after which it stops obeying any command you've given it. To maintain control of the swarm for another 24 hours, you must cast this spell on the swarm again before the current 24-hour period ends. This use of the spell reasserts your control over up to three swarms you have animated with this spell, rather than animating a new one.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, you animate or reassert control over one additional undead swarm for each slot level above 2nd. Each of the swarms must come from different corpses or piles of bones.