The Swarm is the collective name given to the undead remnants of Humanity who were wiped out during the Apocalypse and reanimated by Abyssal Creature's. Individually, they pose little threat but they attack in hordes to overwhelm their prey through sheer numbers. Although they are mindless and feral, they are utilized by the Destroyer's legions in their conflict with the Hellguard. Both Fury and Death are forced to confront these twisted horrors whilst exploring the ravaged Earth during their respective quests.
It is revealed by Uriel that the Swarm are corpses who have been infected by a Suffering. These corpses become extremely savage and will attack anything in sight, although they appear to ignore other demons. They are seemingly drawn to loud noises which is how Suffering are able to direct them with their roars. Whilst Death is travelling through the destroyed city, a large droning noise signals an attack by these creatures.