In Matthew 8:28-34 Jesus casts demons out of a pair of men and into a herd of pigs.

Verses 29-32 are particularly instructive regarding the character of demons:

‘”What do you want with us, Son of God?” they shouted. “Have you come here to torture us before the appointed time?”  Some distance from them a large herd of pigs was feeding.  The demons begged Jesus, “If you drive us out, send us into the herd of pigs.”  He said to them, “Go!”’

A number of points emerge:

1. They know who Jesus is. They know that he is the Son of God.  In this regard they are better-informed than the religious leaders of the day.
2. They acknowledge Jesus’ authority over them. They may victory over the men they have possessed.  But they are powerless in the presence of Jesus.
3. They know that they are defeated. they know that there is an appointed time for their destruction.  Still, like fatally-wounded wild animals, they will not go down without a fight.  To that extent, they are still dangerous.
4. They are knowing, but not all-knowing. They are intelligent.  They know the Bible, and can quote it when it suits them.  They can watch people and events and draw conclusions (is that how they knew that Jesus was/is the Son of God?)  But they are not omniscient.  Only God is.
5. They are not merely evil ‘forces’ or ‘influences’. Like persons, they have will, emotion, and intellect.  But they are not persons.  Probably, they are fallen angels.
6. They are in willful rebellion against God. And so their fate is sealed.

Summarising this article by Brian Dembowczyk