A fallen angel is an angel that has been banished from Heaven because of their disobedience to the orders of God.
Lucifer was a very beautiful Cherubim that because of his pride he rebelled against God and wanted to be like Him. It is said that he was the most beautiful angel that God created. After his offense, he was exiled from Heaven and one third of the angels created by God followed him. Those were the fallen angels. They were thrown from Heaven to a pond of fire and sulfur.
There is the theory that says that Satan can enter and exit Heaven until the resurrection of Christ in which the almost definitive battle between the fallen and the angels happened, and the gates of Heaven were closed for them completely.
The same way that there is a hierarchy for the angels of God, there is also one for the fallen angels.
Satan divided his forces in principalities. Princes of the darkness that will rule on the land and influence and tempting the Humanity.
There is also the authorities and governors of the darkness, demons and malign spirits that influence the social and cultural system.
There are also the fallen angels that defend and worship Satan and will operate in the religious structure of humanity.
The hierarchy of angels is very rigid, each one is in charge of a task. This doesn't seem to happen in the world of fallen angels, were they can change their tasks.
These evil angels may be responsible of tempting with impure thoughts, depression, acting to weaken the mind of people.
Other spirits may afflict the physical bodies bringing disease and temptation.
Other malign spirits may be in charge of seducing mankind and afflict their minds with false doctrines full of confusion.
The fallen angels or demons follow the orders of Satan. Their mission is to go against God's will and the order of God.
The same way that God sent his son Jesus to Earth, Satan will send his son the Antichrist in human shape, who will contradict the word of Christ creating a doctrine that negates Christ.
The main reason of disobeying God is the wish of Satan of being worshiped by all creatures.
The same way that there is a trinity of Father, Son and Sacred Spirit, there is an opposite trinity: Lucifer, Antichrist, and the False Prophet.
The same way that there are angels with hierarchy there are fallen angels of similar rank:
- Astaroth is the archduke of Hell.
- Beelzebub means "the lord of the flies", and is the prince of demons. His mission is to tempt.
- Mephistopheles is a subordinate of Satan in charge of capturing souls.
- Belial is the devil of arrogance and pride.
Angels and Fallen Angels all come from the same source, created by the same hand, although at some point they take different paths. God created them, and they chose.