In Mormon theology, Satan, or Lucifer, was one of Heavenly Father’s firstborn spirit children and is therefore sometimes called a “fallen angel.” The Pearl of Great Price, which stands with the Bible, the Book of Mormon, and the Doctrine and Covenants as one of the canonized works of LDS scripture, offers an account of Satan’s rebellion. God had created a plan of salvation, to be implemented by his son Jesus Christ, that enabled human beings to retain their agency and choose light over darkness.
However, knowing that people would sin and not be able to return to God, Satan rebelled against God by proposing another plan that ensured that not a single soul would be lost. (The catch was that people would have no freedom not to obey and would also be forced to worship Satan himself.) Since God’s will was clearly to preserve human freedom, agency triumphed, and Satan was banished. But as we can see in the story of Adam and Eve, Satan can still exert strong influence on human beings and attempt to persuade them to his bondage.