"The Devil keeps man from good with a thousand machinations spewed from his belly, so that when a person sighs to do good, he pierces him with his shafts; and when he desires to embrace God with his whole heart in love, he subjects him to poisonous tribulations, seeking to pervert good work before God. And when a person seeks the viridity of virtue, the Devil tells him that he does not know what he is doing, and he teaches him that he can set his own law for himself."
He is described as the "ruler of the demons" and "the God of this Age". In Revelation, Satan appears as a Great Red Dragon, who is ultimately defeated by Michael the Archangel and cast down from Heaven. Satan is also known as the Devil and, although the Book of Genesis does not mention him, he is often identified with the serpent in the Garden of Eden. Satan's appearance is never described in the Bible, but, since the ninth century, he has sometimes been shown with horns, cloven hooves, hairy legs, and a tail, often naked and holding a pitchfork, an amalgam of traits derived from various pagan deities including Pan, Poseidon, and Bes.
Satan is said to also be associated with one of the Seven Deadly Sins being Wrath. He is also one of the Seven Princes of Hell. Before his fall, Satan was a Cherubim and a prince of the choir, showing his status as the supreme Cherub as Lucifer is over the Seraphim. This would explain why he is depicted as having three heads in his imprisonment, showing that after his fall his appearance as a Cherubim became highly twisted and corrupted. However, it is believed that Satan was something else before he was appointed as a Cheribum. He himself states that he is a bit of an enigma. Scholars state that Satan was an ancient spirit or force that existed when the first spark of creation was lit, possibly around or before the Earth was created. He is believed to have once been a wandering spirit with no purpose, confused of his surroundings and existence until God appointed him with being a judge. It was until then that he soon became a prince of the Cherubim.
When God commanded the angels to bow to the first human Adam, Satan (then an angel called Lucifer or Helel) rallied other angels against this command with the belief that angels are above all other creations. He and other "fallen angels" fought against the forces of God and were utterly defeated; as a result, Satan was cast down into Hell. After his fall, Satan became the King of the Inferno and the lord of all demonkind with Lucifer being ranked above him as the Emperor of Hell. Despite having rulership over Hell, he remains imprisoned in the depths of the Abyss.
Contrary to popular belief, after the fall it is believed that Lucifer became a demon when his heart was filled to the brim with corruption and darkness. However, regardless of his appearance, Lucifer remains to be a fallen angel rather than a true demon. He is simply regarded as a demon due to his character and appearance. Satan, however, completely discarded his angelic origins and fully embraced his current status as a demon. Thus, Satan is the first true demon.