The Battle Fly (not to be confused with the Rot Fly) is a large daemonic beast in the service of Nurgle which resembles a giant corpse fly. These creatures are created from favored Beasts of Nurgle, much like their very similar counterparts. 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.
While Battle Flies function similarly to the Rot Flies, they differ in that they don't come from a really upset Beast of Nurgle. They also look much more horrid to look at in contrast to the Rot Fly. Could be because the Battle Fly looks like a deranged Wasp (The worst insect that Holy Terra ever gave birth to) whereas the Rot Fly looks like the traditional fly.
They haven't been seen since the Horus Heresy. Rumors spread that they were replaced by the Rot Flies while others suggest that they are mothballed and put in reserves as a "Just in Case" precaution for Papa Nurgle.