WHAT DO DEMONS DO?
In general, demons, like Satan, attempt to thwart the purpose of God in every way possible
(Dan 10:10-14) “Suddenly, a hand touched me, which made me tremble on my knees and on the palms of my hands. And he said to me, "O Daniel, man greatly beloved, understand the words that I speak to you, and stand upright, for I have now been sent to you." While he was speaking this word to me, I stood trembling. Then he said to me, "Do not fear, Daniel, for from the first day that you set your heart to understand, and to humble yourself before your God, your words were heard; and I have come because of your words. "But the prince of the kingdom of Persia withstood me twenty-one days; and behold, Michael, one of the chief princes, came to help me, for I had been left alone there with the kings of Persia. "Now I have come to make you understand what will happen to your people in the latter days, for the vision refers to many days yet to come."”;
Rev 16: 13-16 “And I saw three unclean spirits like frogs coming out of the mouth of the dragon, out of the mouth of the beast, and out of the mouth of the false prophet. For they are spirits of demons, performing signs, which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty. "Behold, I am coming as a thief. Blessed is he who watches, and keeps his garments, lest he walk naked and they see his shame." And they gathered them together to the place called in Hebrew, Armageddon.”).
In doing this they extend Satan's activity, and the very number of demons doing work all over the world for Satan, is what makes Satan seem to be omnipresent, even though he is not.
In particular, demons can inflict diseases (Mt 9:33) “And when the demon was cast out, the mute spoke. And the multitudes marveled, saying, "It was never seen like this in Israel!"”;
Lk 13: 11, 16 “And behold, there was a woman who had a spirit of infirmity eighteen years, and was bent over and could in no way raise herself up. … "So ought not this woman, being a daughter of Abraham, whom Satan has bound----think of it----for eighteen years, be loosed from this bond on the Sabbath?"”),
they can possess animals (Mk 5:13) “And at once Jesus gave them permission. Then the unclean spirits went out and entered the swine (there were about two thousand); and the herd ran violently down the steep place into the sea, and drowned in the sea.”
and, as I’ve already mentioned , they promote false doctrine (1 Ti 4:1) “Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons,”.
Sometimes God uses demons in carrying out His plans (1 Sa 16: 14) “But the Spirit of the LORD departed from Saul, and a distressing spirit from the LORD troubled him.”;
(2 Co 12:7) “And lest I should be exalted above measure by the abundance of the revelations, a thorn in the flesh was given to me, a messenger of Satan to buffet me, lest I be exalted above measure.”.
This reminds us that even these creatures are under God’s complete and constant control.
Demons also seem to be concerned with the affairs of nations. Daniel 10:13 speaks of "the prince of the kingdom of Persia" who seems to be a spirit being who resisted the coming of an angel to bring Daniel a message. That prince was himself resisted by Michael the archangel, which means that he must have been a very powerful demon.