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 Satan, the Devil, the Dark One, the Voice and the Prince of Darkness, is a fallen archangel, the ruler of Hell and master of demons. He is possibly the first angel created by God.

Banishment from Heaven
As his title suggests, the First of the Fallen was the first angel banished from Heaven and cast down to Hell due to rebelling against his creator for unknown reasons and starting a war in Heaven that he lost. Presumably he is also the first angel created by God. Infuriated by his banishment to Hell, but recognizing that even with all his enormous power, he cannot fight against God himself, the Devil created demons and masterminded dark magic, using his dark spells to hunt for human souls, the purest expressions of God's love, by making bargains with them in exchange for any wish 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, Satan came to claim Willie's soul, while the latter was recording at Moonrise Studios. The First of the Falen'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 of the Falen's powers were greatly enhanced by the Rising Darkness masterminded by Brujeria, a sect of warlocks more evil than Hell itself, who worshiped him as a deity. The Rising Darkness allowed the First of the Fallen 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 Satan'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, but among the oldest, if not the oldest archangel, thus he has terrifying levels of supernatural powers that are only undoubtedly surpassed by God himself. As he is one of the first, if not the first angel in existence, he can be assumed to hold more power than the other archangels younger than him and certainly among the most powerful, if not the most powerful angel ever created.
 * Immortality: The First of the Fallen is beyond age and illness as well as most, if not all forms of physical harm. He is known to be as old as the universe itself if not older. Apart from God himself and the other archangels, it is currently unknown if there exist any method of harming or killing him at this point.
 * Nigh-Omnipotence: As the ruler of Hell and among the first, if not the first angel created by God, the First of the Fallen is unimaginably powerful, having supernatural powers on the level of near omnipotence, and is far more powerful than regular angels and he may surpass even the other archangels to an unknown degree. However, while terrifyingly powerful and almost omnipotent, the First of the Fallen's powers still has limits, as one of the few limitations of his power is that he cannot fight against God directly, as for all of the First of the Fallen's immense power, he is still far surpassed only by God. Despite this, the First of the Fallen still ranks as one of the most powerful beings in the multiverse.
 * Dark magic: The First of the Fallen is known to be the mastermind of dark magic and he is connected to many dark magic rituals and possibly being the source of power used by dark sorcerers to perform their spells. He particularly governs over spells connected to the binding of demons and souls. Being the one who masterminded dark magic in the first place, the First of the Fallen's dark magic is far more powerful than any dark magic user, most likely the strongest, with the only known way to dispel his dark magic spells being to use God's magic power's unique but clearly extremely powerful connection to dark magic.
 * Necromancy/Soul pact: Using his dark magic, the First of the Fallen has the power to bind humans souls directly to Hell by making a deal with them to grant their wish in exchange for their soul.
 * Demonic pact: Using his dark magic, the First of the Fallen is able to bind one of his allied demons to the soul of a sorcerer using magical objects representing a demon to make a deal with him, granting the temporary powers at the expense of giving the demon a chance to possess them.
 * Interdimensional 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: The very voice of the First of the Fallen alone can imbue objects with immensely powerful dark magic, inducing whoever heard it to kill themselves, as when it was recorded on an acetate, said acetate gained the abillity to compel anyone who hears it to commit suicide. He even seems to be able to enchant said object to compel whoever is close to it to play it. Even John Constantine, a highly skilled and knowledgeable sorcerer, had to use God's own magic power to dispose of it by drawing on the disk's own dark magic to send it back to Hell.
 * Creation: The First of the Fallen is so powerful that he was able to create millions of demons.

Season 1

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Season 1

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Season 3

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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.