Scripture clearly speaks of the existence of beings known as "demons", or "unclean spirits." We know they do exist. The Bible says the following things about their characteristics.

1. They Are Spirit-Beings

Demons are disembodied spirits. They have no physical form to them whatsoever.

That evening they brought to Him [Jesus] many who were possessed with demons; and He cast out the spirits with a word, and cured all who were sick (Mat 8:16).

Demons belong to the spirit world. Their only manifestation is the disorders they cause.

2. Demons Are Personal Intelligent Beings

It is clear that demons are intelligent beings—they have the attributes of personality.

Suddenly they shouted, "What have You to do with us, Son of God? Have You come here to torment us before the time?"...The demons begged Him, "If You cast us out, send us into the herd of swine" (Mat 8:29, 31).

Demons can talk, make requests, act in fear, reason, and attempt to persuade. These are all attributes of personality.

3. They Are Unclean, Vicious, Spirits

Demons are hateful creatures.

When He came to the other side, to the country of the Gadarenes, two demoniacs coming out of the tombs met Him. They were so fierce that no one could pass that way (Mat 8:28).

4. Some Are More Evil Than Others

The Bible speaks of degrees of wickedness among the demons. Jesus said.

When the unclean spirit has gone out of a person, it wanders through waterless regions looking for a resting place, but it finds none. Then it says, 'I will return to my house from which I came.' When it comes, it finds it empty, swept, and put in order. Then it goes and brings along seven other spirits more evil than itself, and they enter and live there; and the last state of that person is worse than the first. So will it be also with this evil generation (Mat 12:43-45).

The fact that demons are so numerous makes the work of Satan something that takes place everywhere.

And He asked him, What is your name? And he said unto Him, My name is Legion; for we are many (Mar 5:9).

When the demons were driven into the swine at Gadera, two thousand of them ran into the sea and drowned.

So He [Jesus] gave them permission. And the unclean spirits came out and entered the swine; and the herd, numbering about two thousand, rushed down the steep bank into the sea, and were drowned in the sea (Mar 5:13).

This is another indication the demons are numerous.

6. They Are Messengers of the Devil

Demons are the messengers of the Devil. They do the will of their master - Satan (Mat 12:22-30). Satan is called the head of the demons.

Summary

Demons do exist. They are personal intelligent beings who are evil malicious spirits. There are so many of them that it makes the evil work of Satan going on almost everywhere on the earth.