Swarm Form

You transform a willing creature you touch, along with everything it's wearing and carrying, into a swarm of Tiny Beasts for the duration of the spell. The spell ends if the creature drops to 0 hit points. An incorporeal creature isn't affected.

The caster chooses a swarm type choosing from one of the following - swarm of insects,  swarm of insects (beetles), swarm of insects (wasps), swarm of insects (spiders), swarm of insects (wasps), swarm of rats, swarm of rot grubs,  swarm of bats, swarm of ravens.

While in this form, the target moves as per the swarm type it currently holds. The target can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny creature. The target has resistance to bludgeoning, piercing and slashing attacks and is immune to being grappled, paralyzed, petrified, prone, restrained, or stunned. 

While in swarm form, the target can't talk or manipulate objects, and any objects it was carrying or holding can't be dropped, used, or otherwise interacted with. The target attacks as and has access to any action available to the swarm type it has transformed into but may not cast spells or use any racial or class abilities it might have. The target uses the physical (STR, DEX, CON) ability scores of the chosen swarm type but retains its mental (INT, WIS, CHA) ability scores for the duration of the spell.

The GM might allow you to choose different beast types. For example, if you transform into a swarm of bees you might use  swarm of insects (wasps) or a swarm of crabs might use the swarm of insects (beetles). Swarm of ravens can be used for any swarm of tiny birds.