Rot Flies (Not to be confused with the Battle Fly) are the result of a very angry and very upset Beast of Nurgle. When a Beast of Nurgle gets sufficiently angry (Either due to all its playthings dying from its horrid diseases and weight or being told off by Papa Nurgle himself), it mutates from a cocoon into a nightmarish insectoid-like creature that have a appearance so repugnant it is said to be able to scar the mind. Rot Flies are among Nurgle's most loathsome creations, and only the Black Library of Chaos speaks of the process by which they are created. Rot Flies are fucking nasty to say the least.
Rot Flies are often used as mounts by the high-ranking Plaguebearers who form squadrons of aerial cavalry with their Rot Fly steeds known as Plague Drones. The Black Crusade supplement Tome of Decay claims Nurgle assigns them Plaguebearer riders because he doesn't want to see his children suffer, and thus gives them an eternal companion to ease their bitter hearts. Aww...
In terms of its role in war Rot Flies act as fast aerial moving beasts that dives down on unsuspecting prey. As the Rot Flies fall upon their prey, leathery wings buzz in a flapping purr of motion and clouds of deathbottle flies fill the air above, choking airborne warriors and clogging engine intakes. Prehensile probosces and posterior mouth-parts latch onto the faces of their victims, and the Rot Flies let out titters of mean-spirited laughter as they pluck heads from necks and swallow them down.
When facing the common soldiery of Realspace, a Rot Fly will slowly digest all meat from a skull before extruding a plague-infested death's head that its Plague Bearer rider can hurl at the foe. Given the chance, though, Rot Flies will hunt down the heroic warriors that slew their previous incarnations. A special fate is reserved for such individuals; by opening their maws wider than physical law should allow, the Rot Flies swallow their prey whole, keeping them trapped in their mucous-filled abdomens for eternity.. Or at least until the badass in question pulls the trigger. Yeah, eating some sort of extraordinarily awesome warrior whole is a really bad idea.