Rot Flies were once Beasts of Nurgle that have since been transformed by the will of the Lord of Decay into hideous daemonic insect steeds similar in appearance to giant corpse flies whose appearance is so repugnant it scars the mind.
Rot Flies are colossal Daemons in the form of hideous insects whose appearance is so repugnant it scars the mind. These vile creatures are the most loathsome of Nurgle's creations. Only the forbidden tomes of the Aeldari Black Library speak of the forbidden process by which these creatures are birthed, for they hatch in the sticky depths of Nurgle's gardens in the Realm of Chaos, where the visionary and the loon wander in their dreams.
Some Beasts of Nurgle, disappointed by the rag-doll inactivity of their mortal playthings, develop a kernel of bitterness in their ebullient souls. Crestfallen puzzlement leads to frustration and ultimately an aching resentment. Over the millennia, a thin seed of malice grows in such a Beast's heart, feeding upon the energies of its depression and angst until it throbs like a canker.
The final straw comes when the Beast is betrayed unto death by those it wishes to call its friends. Seeking reconciliation, the Beast will put aside its doubts and bound optimistically towards the ranks of those mortals it has cornered.
Should one of these ingrate warriors slay the Beast with a lucky sword thrust or well-aimed plasma blast, the creature will vanish howling into the Warp.
Called back to the Immaterium, the Beast lollops and huffs, splashing down into the mire of Nurgle's garden with an aggrieved sigh. The knowledge that it cannot return to the delights of the mortal realm festers within it as it wallows in the sheer unfairness of the universe.
Over the centuries the Beast pupates, protected from harsh reality by a crawling shroud of Nurgle's fattest flies. A daemonic metamorphosis takes place as the chitinous nub of hate that lurks within the Beast grows strong on the sallow bulk of its former incarnation.
Eventually, the creature within bursts out of its cocoon as a full-grown Rot Fly, a creature of pitiless malice hell-bent on wreaking its revenge upon an uncaring universe. Plague Bearers prize such steeds highly, for in their haste to punish the mortals that once spurned them, Rot Flies speed into battle at a great pace.
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 -- 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.
- Codex: Chaos Daemons (6th Edition), pg. 49
- Codex Heretic Astartes - Death Guard (8th Edition), pg. 55
- White Dwarf 399 (March 2013), pg. 4
- Games Workshop Catalogue: Plague Drones of Nurgle