Grants two Flies that orbit Isaac, blocking enemy shots that hit them. These kill other flies that touch them, but do not damage other enemies.
Can stack with a Pretty Fly/Cube Of Meat/Guardian Angel/Itself up to a maximum of three. Flies have the lesser priority regarding orbitals and are replaced by other orbitals when all three "orbital slots" are taken. Flies cannot replace other orbitals and nothing happens if picked up when already having three.
If Isaac picks up a Pretty Fly while having three orbitals which one of them is Cube of Meat, picking up a third Cube of Meat will cause the "extra fly" to take effect and replace the orbital Isaac lost.
Guardian Angel - The potential to block shots is greatly inceased.
Triviade:Halo Of Flies Modifier
This item is probably a reference to Alice Cooper's song "Halo Of Flies" from the album "Killer".