Holy water

Holy water is weakness that serves to demons, that is used frequently by hunters and agents.

Effects
Holy water burns demons that come into contact with it, like acid (though it does no damage to the human host, only the Demon). It has since been proved to be very effective against demons.

Immunity
The oldest and most powerful demon, Demonic lords are immune to holy water, and the youngest demonic lord, (name unknown) still was immune to it but displayed slight pain.