Often harbingers of famine and decay, these swarms of flies spread disease and pestilence wherever their buzzing wings carry them. These insects typically form a cloud 20 feet across, made of tens of thousands of flies. This cloud moves at up to 10 feet per round, and obscures all sight (including darkvision) beyond 5 feet. Creatures 5 feet away have concealment (20% miss chance), and creatures farther away have total concealment (50% miss chance, and the attacker can’t use sight to locate the target). Moderate or stronger winds can temporarily disperse the cloud, as per obscuring mist, but the flies reform 1d4+1 rounds later to continue their pursuit of carrion.

Spells such as fireball, flame strike, and wall of fire, as well as similar area spells, destroy the cloud of flies if they deal at least 10 points of damage. Creatures that spend at least 1 round in the cloud must succeed at a DC 13 Fortitude save or contract the shakes. Some plagues of flies carry other virulent diseases instead, which might affect the CR of the hazard if the DC is significantly higher or lower.