Demons & Fallen Angels
What does scripture say?
Some excerpts from James M. Gray, D. D. Book: Spiritism and the Fallen Angles. 1920
Angel in Hebrew Malek, in Greek angellus, both meaning messenger or agent.
Angels are spirits, superior to men and inferior to God. Psalm 8:4,5; Hebrews 1:7,8
They have bodies of some kind, Luke 20:36 and they have appeared in human form            (always as men, not woman), and have eaten food and lodge in houses Genesis 18:8; 19:3.
The number of angels is very great Daniel 7:10; Psalm 68:17; Matthew 26:53, and their power likewise, whether exercised in the material or spiritual world Psalm 103:20; II Kings 19:35; II Thessalonians 1:7. And yet their power and knowledge are both limited Daniel 10:13; Matthew 24:36; Ephesians 3:9,10.
Good Angels
Angels are employed both in heaven and on earth. In heaven they are worshippers, Matthew 18:10; Revelations 5:11, but on earth they are associated with the affairs of providence, doing good to God's people and doing judgment on God's enemies, the latter ministry to be intensified in this latter age. II Kings 19:35; I Thessalonians 4:17; Psalm 90:10-12; Psalm 91:11; Mark 13:27.
There appear to be graded positions and authority among them judging by the allusions to thrones, dominions, principalities and powers Ephesians 1:21 and by the mention of Michael as the Archangel Daniel 10:13, and Gabriel as one who stands "in the presence of God" Luke 1:19.
We are not to worship or pray to angels or anyone else in heaven other than Yahuweh through Yahushua.  Colossians 2:18
Evil Angels
AKA evil spirits Judges 9:23; Luke 7:21, unclean spirits Matthew 10:1, and demons Deuteronomy 32:17; Matthew7: 22. Demon is rendered devils in KJV.
We have a basic understanding about Satan and his fall. We will discuss his demons, which abide upon the earth, fallen angels, and their effects on man.
Genesis 6:1,2-4 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, 2 that the sons of God (not men) saw the daughters of men that they were fair; and they took them wives of all which they chose.
4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men, which were of old, men of renown.
These angels (sons of God) Jude 6 And the angels which kept not their first estate, but left their own habitation, He hath reserved in everlasting chains under darkness unto the judgment of the great day.
II Peter 2:4-5 For if God spared not the angels (sons of God) that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment; 5 and spared not the old world, but saved Noah the eighth person, a preacher of righteousness, bringing in the flood upon the world of the ungodly;
We can see from the previous scriptures, these angels (sons of God) are in chains. Their offspring were destroyed buy the flood? Yet the son of Noah or grandson from Ham or most likely Ham's wife iniquity passed on to the sons of Ham and His offspring were cursed specifically the Canaanites. Genesis 9:25 This did not restrict individual salvation, Rahab join Israel and the King of Tyre, Hiram, gave materials for the temple.

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The leader of evil angels is Satan, which are many according to scripture enough to compose a kingdom with Satan as their leader. Luke 8:30; Matthew 12:24-26; Ephesians 6:12
These demons are called Satan's agents in Matthew 12:26,27; 25:41; and as such able to inflict physical maladies on men Matthew 12:22; 17:15-18; Luke 13:16; and even enter and control their bodies and those of beasts also Mark 5:13; Acts 16:16; while from a moral point of view they are able to seduce men from the truth and lead them into all uncleanness. I Kings 22:22; I Timothy 4:1; II Peter 2:10-12.
It is important to note that these demons maintain a conflict with the followers of the Messiah Yahushua (Jesus). Ephesians 6:12, and God the Father, Yahuweh, sometimes uses them in judgment upon unbelievers and wicked men. Judges 9:23; I Samuel 16:14 They are to be used in the awful judgments upon the earth during the tribulation period Revelation 9:1-11; 16:13,14; but their own eternal fate, like that of their mighty but unholy leader, is one of torment Matthew 8:29; Luke 8:31.
What is true of the leader is of the followers so let us look close at Satan. Taken from Dr. Gray's book " Spiritism and the Fallen Angels" chapter III
Even the casual reader of the Bible will recall the outstanding occasions when Satan appears on its scenes. First, as the "serpent" in the Garden of Eden tempting and overcoming our first parents, for twice in the New Testament the serpent is identified with the devil and Satan Revelation 12:9; 20:2. Next, in the history of Job chapters 1 & 2; than later, in vision, in the Old Testaments the accuser of Israel after their return from Babylon Zechariah 3.
In the New Testament he first comes before us as Messiah's tempter in the wilderness Matthew 4 and its parallels; he entered into Ananis and Sapphira to lie to the Holy Ghost Acts 5:3; He hindered Paul in his missionary work I Thessalonians 2:18; he is said to walk about as a roaring lion seeking whom he may devour I Peter 5:8, and in the end of this age he will give his power to the Lawless one to deceive, if possible, the very elect Matthew 24:24, II Thessalonians 2:8-10; Revelations 8:1,2.
Character of Satan
Many times in Scripture Satan reveals a part of his character. Satan means pronounced in Hebrew (sawtan) to withstand, oppose, adversary, arch enemy of good; devil, (slanderer); Apollyon, (destroyer); Beelzebub, (prince of demons); belial, Low, abject); "the wicked one"; "the god of this world"; "the prince of darkness"; "the dragon"; "tormentor"; "accuser"; "deceiver"; "liar"; "murder"; "he that hath the power of death." This sufficiently reveals the kind of being he is.
Origin
As to the origin of Satan, those who class the cherubim as angels, think he was one of them, and anointed probably for a position of great authority. This authority may have been over primitive creation, i.e., heaven and earth as at first created, and before the earth became without form and void Genesis 1:1.
He fell through pride Isaiah 14:12-14. Some think he was cast out of heaven according to Messiah's words in Luke 10:18, on which supposition, he then made the earth and the air the scene of his activity Ephesians 2:2; I Peter 5:8.

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A scriptural basis for this conjecture is in Ezekiel 28:11-19.  The chapter is a rebuke of the King of Tyre, but at the verses named the language goes beyond him to Satan described in his unfallen state.
If this interpretation is correct, the chapter teaches much about Satan, which is summed up by Dr. R. A. Torrey in "What the Bible Teaches," thus:

1. He was the sum of created perfection, v. 12.
2. He was in the garden of God, v13. This does not mean the Eden of Adam, but an earlier one. The Eden of Adam was remarkable for its vegetable glory, but this for its mineral glory. Cf. Revelation 11:10-21.
3. He was the anointed cherub that covereth, v.14. First he was a "cherub", the highest rank in the angelic world. Second, "the anointed" cherub, i.e., one who was set apart for a formal work. Third, the anointed cherub that "covereth." The meaning of this word is not given, though it suggests Exodus 37:9.
4. He was upon the holy mountain of God, v.14, which may mean the place where God manifest His personal glory.
5. He walked up and down in the midst of the stones of fire, v.14. This suggests Exodus 24:10,17, R.V.; Ezekiel 1:15,22,25,26, R.V. In the former the seventy elders saw Yahuweh of Israel, and "there was under His feet as it were a paved walk of sapphire stone", and "the appearance of the glory of Yahuweh was like devouring fire on the top of the mount." This may afford an idea of what the "stones of fire" were, and indicate how near Satan may have been to God.
6. His heart was lifted up because of his beauty, v.17. Cf. I Timothy3: 6.
7. He was cast out of the mountain of God, and destroyed from the midst of the stones of fire, v.16.
8. He shall be cast to the earth and be made a spectacle, v.17, and 18. Cf. With II Thessalonians 2:8-10; Revelation 12:9,10; 19:20.

Further nature and character

1. He is a person, i.e.; he possesses self-consciousness, and free will, because the facts stated in the preceding paragraphs could not be predicted of an influence or a principle of evil.
2. He has great dignity, since he is styled the "prince," "the god of this world," and "prince of the power of the air," and it is said that Michael the Archangel "durst not bring against him a railing judgment," John 12:31; II Corinthians 4:4; Ephesians 2:2; Jude 8:9, K.J.
3. He has great power, since he is able to control the forces of nature, human property and life, demons, the world-rulers of darkness, and the whole world of men out of Messiah, Job 1:10-12; Luke 11:14-18; I John 5:19; Acts 26:18; Ephesians 6:11-12.
4. He has great cunning and deceit, since he transforms himself into an angel of light, uses wiles and devices, signs and lying wonders Matthew 14:24; II Corinthians 2:11; 11:14; Ephesians 6:11,12; II Thessalonians 2:9.
5. He has great malignity, being called the evil one, a liar, a murder, and a sinner from the beginning Matthew 5:37; Luke 8:12; John 8:44; II Corinthians 4:4; I John 3:8.
6. He has great fear, for if we resist him he will flee from us James 4:7. 

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Location and work
He is referred to as being in the heavenly places, and also as going to and fro in the earth. As in the age to come, Messiah and His Church, though abiding in the heavenly places, will rule an earthly people, so now Satan and his hosts abiding in the heavenly places are ruling an earthly people. Job 1:6-7; Ephesians 6:11-12; I Peter 5:8; Revelation 12:9.
His work:

1. He is the author of sin and tempts to sin Genesis 3:1-6; I Chronicles 21:1; Matthew 4:1-11.
2. He produces sickness and has the power of death Luke 13:11-16; Acts 10:38; Hebrews 2:14.
3. He lays snares for men I Timothy 3:7.
4. He takes the word of God out of their hearts Matthew 13:19.
5. He puts wicked purpose in their hearts Ephesians 4:27.
6. He blinds their minds II Corinthians 4:4.
7. He harasses and accuses them II Corinthians 12:7; Revelation 3:9.
8. He enters into them John 13:27
9. He does this and more by the angelic messengers who carry on his work II Corinthians 11:14-15; Revelation 3:9

Happily however, there are certain limitations of his work and power. For example, being a finite and created being he can be only in one place at one time, though, what is done by his agents, being attributed to him, he is practically ubiquitous.
In the second place, it is reassuring to know that his influence over the bodies of men is entirely subject to God's control Job 2:7; Luke 13:16; Acts 10:38, and that his influence over the souls of men is simply, oral. That is to say, he may offer suggestions and deceive or persuade men, but he is absolutely unable to change their hearts or coerce their wills. "Every man is tempted, when he is drawn away of his own lust, and enticed" James 1:14.
Our responsibility in regard to Satan therefore. Is threefold: We are to watch against him I Peter 5:8; to give no place to him Ephesians 4:27; and to resist him James 4:7.
We watch against him as the context of the passage shows, by humbling ourselves under the mighty hand of Yahuweh and casting all our anxiety upon Him. We give no place to him by restraining wrath and eschewing falsehood; and we resist him by the Sword of the Spirit, which is the Word of Yahuweh.

Key Scripture to remember whose you are:
I John 4:4 You are of God, little children and have overcome them: because greater is He that is in you, than he that is in the world.
John 15:16-17 You have not chosen Me, but I have chosen you, and ordained you, that you should go and bring forth fruit, and that your fruit should remain: that whatsoever you shall ask of the Father in My name, He may give it you. 17 These things I command you, that you love one another.

Halle lu Yah to Yahuweh in the name of Yahushua