Satan

"The Devil used to be an angel. That's why he's called the "First of the Fallen." You ask a holy type, and they'll tell you... the soul is the purest expression of God's love. The spark of creation. Every time the First takes a soul, he's exacting revenge, paying back the Almighty for casting him out."

- John Constantine

The First of the Fallen, also known as the Devil, the Dark One, the Voice, the Prince of Darkness or Satan, is the ruler of Hell and master of demons.

Banishment from Heaven
As his name suggests, the First of the Fallen was the first angel banished from Heaven and cast down to Hell. Presumably he is also the first angel created by God. Angered by his banishment to Hell, but unable to defeat God himself, the Devil started a hunt for souls, the purest expressions of God's love, so that every time he took a soul, he'd exact revenge on God.

The Devil's Vinyl
A Blue's musician, by the name of Willie Cole, sold his soul to the First of the Fallen for fame. After a period of time, the First came to claim Willie's soul, while the latter was recording at Moonrise Studios. The First's voice ended up being recorded on the song Willie was recording at the time. Anyone who picks up the record will become tempted to play it, and if they listen to the Voice, the person will fall prey to the Devil.

Dark pacts and the Rising Darkness
The First's powers were greatly enhanced by the Rising Darkness masterminded by Brujeria, a sect of warlocks more evil than Hell itself, who worshiped the First as a deity. The Rising Darkness allowed the First to release more and more demons and evil gods upon Earth, as part of the Brujeria's plan to combine Earth and Hell into one dimension. John Constantine, who confronted the Brujeria, was left at the mercy of Invunche and had to make a deal with the First of the Fallen to save his life. Using the Icon of Pazuzu, he removed his protective wards and welcomed the demon king Pazuzu into his body, regenerating it and saving John from Invunche. However, as Manny explained, John was ultimately tricked by the First of the Fallen, accepting a pact for temporary power in exchange for losing his soul to the demon binded to his body by the Devil's dark magic. Although Constantine escaped such grievous fate, he encountered another of the primordial demons: Nahash, the Tempter, the Original Serpent from the Garden of Eden.

Powers

 * Angelic physiology: the First of the Fallen is a fallen archangel, thus has immense supernatural powers that are exceeded only by most other archangels and God himself.
 * Immortality: Due to him being the First of the Fallen and presumably the first angel created by God, he is beyond age and illness as well as most, if not all forms of physical harm. It can be assumed that the only ones that can kill him are God himself or maybe other archangels.
 * Semi-Omnipotence: As the ruler of Hell and presumably the first angel created by God, the First of the Fallen possesses near-omnipotent levels of supernatural powers, that surpasses that of regular angels, and most other archangels; his powers are surpassed only by God himself.
 * Dark magic: The First of the Fallen is the mastermind of dark magic, particularly spells connected to the binding of demons and souls.
 * Necromancy/Soul pact: The First of the Fallen possesses the power to bargain with humans for their souls, in exchange for a wish.
 * Demonic pact: The First of the Fallen may bind one of his allied demons with a bargaining sorcerer, granting the summoner temporary powers at the expense of giving the demon a chance to possess them.
 * Dimensional travel: The First of the Fallen is able to travel from Hell to Earth, in order to claim the souls of those who sold theirs to him.
 * Mind control: When the voice of the First of the Fallen was recorded on a record, people who listened to it were induced to commit suicide.
 * Creation: The First of the Fallen is so powerful that he was able to create millions of demons.

Behind the scenes

 * In DC Comics, particularly Vertigo Imprint, "the First of the Fallen" and Lucifer are treated as two separate characters. First of the Fallen, the original Satan, was a being created by God to act as His conscience. However, the First started to believe that God was insane, and as a result was banished from Heaven and cast down to Hell, becoming the first of the many other angels would fall, creating a Triumvirate of Hell with the Second and the Fourth great fallen angels, assuming that they shared his own power. Lucifer Morningstar was the rebel archangel who lead the revolt of angels against God and was the fourth being to banished. Created by God to be the most powerful angel in the universe, Lucifer deposed First of the Fallen and took over as Samael/Satan, ruler of Hell. He later abdicated, giving the throne back to the First. Both characters shared the titles of "the Devil" and "Satan" at times.