A Death Fly is a flying shadow spirit that can inflict a permanent, irrevocable damage to the victim's Strength. Since only Evolution can escape the crippling damage by transforming, Death Flies become a top priority target as enemies. As an ally, a Death Fly can whittle a hard-hitting foe down even if it can withstand continued hits.
A Death Fly at its max level can evolve further into a Shadow Fairy.
Death Flies is absolutely terrifying as enemies and mildly unsatisfying as allies. Despite its powerful magic growth, a Death Fly does not learn any skills to use it innately. ShadowFlight in combination with ShadowBond can transport a slow, hard-hitting shadow unit into or behind enemy lines, however.
It may be worth using a Death Fly to attack directly if an attrition battle is expected. Their Decay passive works against enemies just as well as against your allies. Its actual hits will not be significant, but the victim will be dealing less and less physical damage over turns.