The Spherewalker Prestige class (5 levels) has a capstone that lets you turn into a swarm of butterflies and still cast spells. It's very much Desna based but has no limitations as to who can take it.
Swarm Form (Su)
At 5th level, a spherewalker gains the ability to transform into a swarm of Diminutive butterflies. In swarm form, she has a space of 10 feet (roughly filling the entire area) but can shape this space to fill four contiguous squares (such as a 5-foot-by-20-foot line, an L-shaped cloud, and so on) and can squeeze through any space large enough to contain one of her component forms. The swarm can fly at a speed of 40 feet (good). Like any swarm, it can occupy the same space as another creature regardless of its size.
Any creature that begins its turn sharing a space with a swarm must succeed on a Fortitude save (DC 10 + spherewalker level + Constitution bonus) or be nauseated for 1 round. Unlike most swarms, a spherewalker in swarm form does not do swarm damage to creatures she’s swarming over. The swarm form is immune to weapon damage but is vulnerable to mundane fire attacks (torches, alchemical fire, burning oil, and so on), and energy attacks from weapon (such as flaming and frost) deal full damage even though the basic weapon damage has no effect. Although a swarm cannot make attacks, the spherewalker can cast spells as normal while in swarm form (although spells with material components could prove difficult).
A spherewalker can use this ability a number of times per day equal to her class level and remain in this form for up to one minute. While in swarm form, she may expend one use of this ability as a free action in order to remain in swarm form for an additional minute (rather than changing back to her normal form and activating it again). Changing back to her natural form before the effect ends is a standard action.
So what happens if you put enough creatures in the same square? Such as getting 10-20 bats/rats/ect and assuming said form as per wild shape?
Good luck doing that. Note that a rat swarm (Tiny) contains 300 rats, and a bat swarm (Diminutive) contains 5000 bats.
A swarm of Tiny creatures consists of 300 nonflying creatures or 1,000 flying creatures. A swarm of Diminutive creatures consists of 1,500 nonflying creatures or 5,000 flying creatures. A swarm of Fine creatures consists of 10,000 creatures, whether they are flying or not. Swarms of nonflying creatures include many more creatures than could normally fit in a 10-foot square based on their normal space, because creatures in a swarm are packed tightly together and generally crawl over each other and their prey when moving or attacking. Larger swarms are represented by multiples of single swarms. The area occupied by a large swarm is completely shapeable, though the swarm usually remains in contiguous squares.[/quote