They are flies, but they have a sucker like a mosquito. They use it to suck the body of the rotting flesh, or to sting the living and it's really painful. They are two to three times bigger than a normal fly.
Death flies can smell rotting flesh far away from them, they just suddenly appear. Their suckers are also stingers. Death flies are able to summon a whole swarm of them in just minutes.
It is the manifestation of Viktor's obsession with flies.