Lucifer was an archangel who once sat at the right hand of God. Jealous of God's power as the Lord of All Creation, he sought to dethrone God. He failed to attain that lofty goal and was removed from Heaven.
History[]
Some time after God created the universe, He created the Celestial Filing Depository. None of the angels ever thought to ask why. Since no one was really in charge, everyone just did their thing, and it was very disorganized. This is when Lucifer stepped up and took control. He created a framework and power structure, not unlike a corporation, and positioned himself as the chief executive officer.
Soon though, Lucifer became dissatisfied with simply being in charge of the universe and decided he wanted to own all that power. He orchestrated what would one day be called a hostile takeover, and he went to war against God and Heaven. He lost. God had seen the signs and knew what Lucifer was up to, and He took steps. Afterwards, God cast Lucifer from Heaven.[1]
The defeat and humiliation stung, so Lucifer, along with the angels who had followed him out of Heaven, went to Hell where he set up a rival filing depository. Meanwhile, God took steps to ensure that this couldn't happen in Heaven again.
Lucifer had a brother in Heaven who had become jealous of the power Lucifer was wielding, so when the war began, he sided against Lucifer. That brother's daughter is Cassiel.[2]