Wondrous item, very rare
This crystalline nest is three inches across and sits comfortably in the palm of the hand. If you put your ear to it, you can hear hundreds of voices arguing in a din of shrill whispers. It was made for Andelbrand, the bastard king of Dorgard, by an elf witch whose name is lost to history. The stories tell that Andelbrand spent all his time with the ancient witch, who fawned over him as a surrogate mother and showered him with sorcerous gifts. When his queen committed suicide, he announced to the kingdom that he would take the elf-crone as his bride. That night, the commoners stormed the castle and burned him alive with his betrothed.
When you cast find familiar through Andelbrand's Nest, the familiar emerges from the nest in one of the following forms: a swarm of bats, swarm of cats, swarm of insects, swarm of poisonous snakes, swarm of quippers, swarm of rats, or swarm of ravens. The familiar swarm can spend 5 feet of movement to break up to five individuals away from the swarm, which use the statistics for a single creature of that type (bat, cat, raven, etc.). All of your familiars act on the same turn, and you cannot have more than twelve familiars divided from the swarm at once.