Miasma

Miasma is a black fog that fills the air of Hell and the bloodstreams of Demons. It is the reason why Demons can enter Demon Form. When in Hell, all Demons and Half-Demons have to do is take a deep breath to enter Demon Form. Miasma is highly toxic to Angels, which is why they can't enter Hell. Absorbing even a little bit will completely disintegrate an Angel.