The Fall of Satan
Very little is known about the fall of satan.
We know that before the Creation, there was war in heaven.
And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, and prevailed not; neither was their place found any more in heaven. And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
- Revelation 12:7-9 (KJV)
We know that satan wanted to be like the Most High, e.g. his sin is that of coveting - wanting to be like God - wanting to be worshiped.
And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand.
- Revelation 14:9 (KJV)
And that's essentially "the great controversy". Who do you worship?
Ellen White has a lot more to say about the fall of satan.
Check out this article about the fall of satan (Ellen White, Signs of the Times).
Also check out this article for more about the fall of satan, and what he actually looks like (Ellen White, Early Writings).