A fallen angel is a former messenger of God who has either willingly forsaken their role or has somehow become corrupted. As a result these angels have been cast out of Heaven and either taken resident on one of the many depictions of Hell or have taken to wandering among the mortal races.
Fallen angels are normally percieved as demons, evil, and unclean spirits. Their lust for power is matched with an evil mind, emotions, and will. In their present, restrained state, these demonic angels are awaiting God’s final judgment and their ultimate destination; a predestined, eternal hell, a place of “reserved everlasting chains.”
Some stories tell of how interactions between fallen angels and humans led to the creation of superhuman beings akin to giants, who were eventually destroyed by God either via the Flood or some other event.
Regardless of where the fallen angels were condemned to they are believed to have lost their link with the ideals of their particular faith, causing them to work against the forces of good and side with evil.
It is believed that these angels fell from grace after following Lucifer or Satan during his rebellion against God. It is even stated that Lucifer managed to convince a one third of the angelic host to side with him in the rebellion, thus, there are currently a one third of fallen angels that dwell within the pits of Hell. After the fall, they seemingly lost their angelic power and status, and were twisted into that of demonic. Their demonic power is said to be just as great as when they were angels, however, despite this they remain to be restrained under Lucifer's power.