The Swarm is a gargantuan hive mind composed of billions, maybe even trillions, of insects inhabiting the Western Forest. It is highly intelligent though its alien thought processes often makes it appear to have an almost childlike naivety. It normally communicates (or attacks) by taking on a humanoid form composed of thousands of insects, though these swarms may take any shape they see fit.
The biology of the Swarm remains largely unknown. The majority of the whole appears to be composed of the flying drones which construct the towering hives that the Swarm inhabits. These hives are guarded by a larger, flightless 'soldier' variant and house the royalty (if the Swarm even have a social hierarchy) of the Swarm; the Patriarchs. These giant centipedal creatures seem to act as some sort of nexus for the Swarms distributed mind and incubators for its countless eggs.
The greatest of the Swarm hives is the The Citadel of the Swarm located deep within the Great Forest, what it may house is a mystery.
More recently the Swarm has made efforts to open up diplomatic relations with the White City; first through emissaries and then through the construction of an embassy.