Lucifer: a 'fallen' angel. Opinions differ as to the precise nature of what happened in the past that could have caused an angel to fall, but what is unarguable is the persistence with which this idea has shown up in most planetary belief systems.
In Roman astronomy, Lucifer was the name given to the morning star (the star we now know as Venus). It is said that Lucifer was one of the seven great archangels of our solar system serving as the guardian of the planet Venus. So the story goes, God asked for a volunteer from among his top angels who might be willing to go down to Earth and help strengthen humanity's spiritual resolve by offering constant temptation. It was Lucifer who volunteered. Despite his apparent loving intentions, slowly, over the ages, Lucifer has become identified in our mind as a devil, instead of an aspect of God dedicated to our growth by helping us strengthen our spiritual muscles!
The name Lucifer actually means giver or bearer of light (from the Latin term lucem ferre). One of his tasks is to teach us about the necessary dark side of life. Lucifer is the shadow that reveals the light by contrast. In many ways we can't see the true light unless we first experience the darkness. It seems to be human nature that we tend not to value something until we lose it and then regain it through our own efforts.
This interdependence of light and dark, of joy and sorrow, of good and bad, and all the other opposites in our dual system of reality, yields to an understanding that these dualities are integral parts to the same whole. Finally we seem to be abandoning the concept that there is a real devil and slowly and surely, we are collectively emerging from the illusion of evil. Part of our growth is to come more fully to terms with the shadow side of our own nature. As we learn to release and finally let go of negative and self-destructive behaviour, we also cease to project our own negativity onto a fictitious devil, or fallen angels, or onto other people. When we reach this point we have no further need to hang onto an illusion of evil.