A Battle Fly is a large daemonic beast in the service of Nurgle which resembles a giant corpse fly. These creatures are created from favoured Beasts of Nurgle much like their counterparts the Rot Flies, which are used as aerial steeds by the Lesser Daemons of the Lord of Decay known as Plague Drones. Battle Flies were first encountered by the Loyalist Space Marine Legions of the Imperum of Man in the early years of the 31st Millennium during the dark days of the Horus Heresy.
- Collected Visions of the Horus Heresy, pg. 329