ephemeral swarms

It’s rare that non-sapient lifeforms become undead after they die, but if enough sentient beasts die at the same time and same place in a painful manner, the psychic agony can spawn a swarm of incorporeal vermin capable of inflicting harm on the living.
Ephemeral swarms inflict roughly the same damage as ordinary swarms of the base creatures type, but because they incorporeal they are very difficult to fight back against without magic.
Ephermeral swarms are difficult to command with necromancy and they all but impossible to command with animal magic. A spell-caster with both necromancy and animal magic can command ephermeral swarms with relative ease but very few spell casters learn both types of magic. Even then, they cannot be controlled so much as directed.
No spell caster has managed a means to reliably create an ephemeral swarm, all known swarms of this type were seemingly created by accident though thereotically if a necormancer were to ruthlessly destroy random living swarms long enough eventually an ephereal swarm might result, few are that patient.