anthropomorphic crows inhabiting the nether.
mob growth as enderman
two-layer texture: the first layer is the body; space instead of head; the second layer is a skull with empty eye sockets; feathers on hands and feet;
screams like the Karelian bittern
becomes invisible, emitting black particles of fireworks; moves faster, trying to go from the rear (smoke particles are still visible).
aggressive to the player and bone mobs.
attack ♥x2 / 3,5 / 5 ( depending on the difficulty)
speed 4 meters per second (8 meters per second in invisibility)
skull of nethermor
maybe something individual and new for netherworld