Demons are obliged to serve God.
They fell before the start of the visible universe, and at that time they lost all the good of their natures. They are the enemies of mankind and sent on the service of vengeance.
“They unsettle the senses, stir low passions, disorder life, cause alarms in sleep, bring diseases, fill the mind with terror, distort the limbs, control the way in which lots are cast, make a pretence at oracles by their tricks, arouse the passion of love, create the heat of cupidity, lurk in consecrated images; when invoked they appear; they tell lies that resemble truth; they take on different forms, and sometimes appear in the likeness of angels.”
Their superiority is increased by their great intelligence which is retained from their angelic nature. Their power of foreknowledge and their great experience makes man’s struggle against them hopeless. They are persistent: the devil never rests from attack on the just man, who is sometimes reduced to the straits of despair.
Pagans of pre-Christian era perceived to be willing victims of the demon who tries to enter the mind of man while sleeping or awake. The Christian man has the angels to help protect him.