Satan is a fallen angel and he used to be God's favorite, until he became the Source of Darkness.
While on Earth, Satan lacked a physical form and only existed in a host. While in control of Jed Potter, his behavior was monstrous and uninhibited, feasting on the blood of animals and chanting dark, unintelligible incantations in dead languages. After Jed was exorcised at Briarcliff Manor, he took control over the body of Sister Mary Eunice, using her innocent guise as a cover to carry out subtle manipulations and mentally torture Briarcliff's inhabitants. He took special satisfaction in mocking Sister Jude, whose dark past involved a drunken hit-and-run resulting in the "murder" of a little girl, as well as Dr. Arthur Arden, whose previous life as a Nazi scientist allowed the Devil to blackmail him into carrying out his commands.
As time went on, Satan eventually chose to reveal himself to Jude, who he had previously encountered during Jed's exorcism, and Dr. Arden, who desperately followed Satan in hopes of seeing at least a glimmer of Mary Eunice, whose innocence he cherished. After Jude was removed as head of Briarcliff and incarcerated within, Satan grew closer to Monsignor Timothy Howard, promising to help elevate him through the ranks and make him Pope. His master plan was thus set in motion: a take over of the Catholic Church.
Personality and Appearance
His true form is a humanoid being with large, bat-like wings and a head reminiscent of a goat, from which two long, twisting horns protrude. Largely cold, emotionless, and sly in disposition, Satan rarely smiled or laughed save for taunting others and mocking the fates of those harmed by his influence. He had very little regard for human life, and had no qualms in murdering and manipulating various mortals in cold blood. He found great satisfaction in humankind's many flaws, and frequently reminded his victims of this in various violent and psychologically harmful ways. He could be perverse and sexually aggressive at times, having made advances on both Arden and the Monsignor, as well as expressing a flirtatious behavior toward Sister Jude. He seemed very comfortable in Sister Mary Eunice's skin, even in private, and used his hold over her body to take away her virginity without her consent, dominating what little bit of her soul still remained in her.
Though cruel and ambitious in nature, Satan was not without his redeeming qualities. He had strong values toward freedom of will and thought, believing firmly that no one should be told who they are or what they should be based on the rules of man and religion, and should, therefore, be able to make their own destinies. Unlike the behavior he exhibited toward adults, he appeared to be very docile around children, calmly and honestly exposing his true identity to Jenny Reynolds and smiling down at her in a friendly gesture. Despite this, the kind of behavior he encouraged in Jenny, as well as other individuals, such as Leigh Emerson, was violent and disturbing, and would thereby contribute to his overall plan of bringing destruction in the faith of God and the downfall of mankind.
In addition to his mind-breaking influence, Satan was endowed with many unnatural, otherworldly powers. Among them, the ability to possess various hosts until the moment of their deaths - only moving on to a new victim when the previous host's body was destroyed - as well as telekinetically moving objects with his mind and reading the minds of all those around him, where he took great joy in seeing other's darkest secrets and manipulating them for his own gain and amusement. Unfortunately, because the Devil thrived solely through possession, he could not escape the kiss of Shachath, the Angel of Death, as failure to escape his previous host's body in time would result in him being cast back into Hell, making the host both his strength and ultimate weakness.
Though long since banished by the Angel of Death to his eternal prison, Satan would continue to influence people for decades to come, accumulating his unseen presence in locations of massive tragedy across the world, manipulating the worst traits of their inhabitants to commit unspeakable acts of violence. Eventually, the Devil saw the perfect formula in Tate Langdon's abusive childhood to possess him into siring the Anti-Christ, Michael Langdon, whose presence would, at last, bring about the Apocalypse.
After initially getting into Briarcliff through Jed, Satan took over young, innocent Sister Mary Eunice, and slowly used malice and manipulation to get Jude fired from Briarcliff.  Acting as Eunice, he replaced Jude as head of the establishment, becoming superior over Dr. Arden. "Mary Eunice" slowly began to get Monsignor Timothy's approval of "her" changes to Briarcliff's conduct and daily life for the inmates. This included a Christmas party, and a showing of "Rudolph the Red-Nosed Reindeer", things Jude had previously described as being acts of turning "away from God". Still, despite the public face as sweet, innovative Mary Eunice, Satan was committing dark, far less subtle crimes in the shadows, such as killing the Briarcliff security guard Frank, setting serial killer Leigh Emerson loose on Jude, and killing a man Jude was using to get information on Arden (the Devil used this murder to reveal himself to Jude). 
At one point, Satan confronted Shachath, the Angel of Death, demanding she leaves Briarcliff after the Angel was "called" by a dying inmate. Shachath reported that she had been invited, unlike Satan, whom she referred to as a cousin - "like Shacath, but fallen". The Angel also noted that she could hear Mary Eunice trying to fight against the Devil, and momentarily, Mary Eunice took control of her own body again, sobbing and pleading to Shachath to free her. The Devil quickly resumed control; before leaving, Shachath warned him they would meet again. 
Soon, the Monsignor learned of Mary Eunice's possession after being told about it by Shachath, and he attempted to cast the Devil out of Mary Eunice. However, the Monsignor was overpowered and raped by Satan, who took his virginity and that of Mary Eunice's. Lost and helpless, the Monsignor consulted with Sr. Jude - now merely Judy, a patient at Briarcliff herself. In her insensible, post-electroshock-therapy state, Judy could only warn that the Monsignor kill Mary Eunice before it was too late. 
Later, Satan confronted Timothy and repeated his promise to manipulate events until Timothy was made Pope so that the Church would finally be under Satanic control. Defying this, Msr. Howard attempted to appeal to any vestiges of Mary Eunice's true self and her inner virtue. This infuriated Satan, who attacked the Monsignor and promised to devour the "last morsel of Mary Eunice's soul". Briefly, Mary Eunice regained control of her body once more, tearfully telling the Monsignor she was tired of fighting against Satan, and that she wanted to "let go".
Taking pity upon the tearful nun and seizing advantage of her weakened state, the Monsignor quickly cast Mary Eunice over a nearby staircase railing. They exchanged one last gaze as the young nun fell before she hit the floor far below. Shachath then appeared, kneeling down before the fatally injured Eunice, who again asked the Angel to take her. "I'll take both of you", Shachath replied and kissed her, releasing Mary Eunice to Heaven and presumably casting Satan back into Hell. 
According to Billie Dean Howard, Satan was able to exert his influence through the negative energy that amassed within the Murder House. This allowed Satan to use the ghostly occupants of the Murder House as his conduits, as it was foretold that the unholy union of a ghost and a human would bring forth the Anti-Christ.  This event came to pass in 2011, when Tate Langdon raped Vivien Harmon in an attempt to provide a child for the grieving Nora Montgomery.  In doing so, he unknowingly allowed Satan to sire the Anti-Christ, who was born in 2012 and named Michael. 
As the Source of Darkness and God's former favorite angel, Satan is one of the most powerful beings in the universe and possesses vast supernatural abilities. He was able to bestow Michael Langdon the powers of the Seven Wonders as well as additional abilities, such as control over the afterlife in Hell.
- Immortality - As a demon, Satan is not susceptible to human weaknesses, thus being immune to all aging, disease, and death.
- Telekinesis - Satan can move objects and people with his mind.
- Concilium - Satan can influence the minds of others through seduction and persuasion.
- Transmutation - Satan can move instantaneously from one location to another.
- Divination - Satan can gain knowledge through indirect means.
- Descensum - Satan can descend into the netherworlds of the afterlife.
- Pyrokinesis - Satan can create, control and manipulate fire.
- Vitalum Vitalis - Satan can balance the scales of life and death.
- Clairvoyance - Satan can enter into the minds of others, read their thoughts, and access their memories, granting him an inherent knowledge of others' past sins, regrets, and moral choices.
- Stiricidium - Satan can control the weather, particularly through molecules of water.
- Illusion Manipulation - Satan can create, shape and manipulate illusions, causing others to see, hear, touch, smell and/or taste things which do not actually exist or cause them to perceive things differently from what they truly are. The Devil use this ability to manifests himself before Michael Langdon by taking on various forms, including a boy and a girl dressed in white, a handsome angel, Anton LaVey, Miriam Mead, and a black goat.
- Satisfying desire - Satan can give everything humans want, such as power, knowledge, wealth and fortune.
- Possession - Satan can possess humans, taking control of a host's mind and body to manipulate for his own purposes.
- Sovereignty of Hell - As the Lord of Hell, Satan rules and oversees all dimensions in Hell.
- Soul Consumption - Satan devours the souls of those he is possessing, and can also cast souls into their personal imprisonments in Hell.
- Immunity to Timeline Alteration - Unlike humans, but like other demons, Satan seems both aware and unaffected by the alteration of timelines.
- Cosmic awareness - Satan knows and understands what's transpiring across the universe.
- To Dr. Arden: "Now... I have work to do, and you're either with me, or you're against me. And if you're against me, even God can't help you."
- To the Mexican: "Why are you so fearful, my child?"
- To Dr. Arden: "You really think you're in charge? You touch me again, you'll die."
- To Jenny: "Poor Mary Eunice! Blah!"
- To Dr. Arden: "Did you celebrate Christmas in your Nazi household?"
- To Briarcliff inmates: "We all are going to be together in the dark watching The Sign of the Cross. A movie filled with fire, sex and the death of Christians... what fun!"
- To Frank: "Now? But the Christians are about to be eaten!"
- To Dr. Arden: "Rubies are the most glamorous of all. You couldn't have found them in this dreary town. Are they family heirlooms?"
- To Dr. Arden: "Jesus Christ... You're being pitiful Arthur."
- To Timothy Howard: "I'm done with you. And with your sweet nun, I'm about to devour the last morsel of her soul!"
- To Jenny: "I know everything. I'm the Devil."
- This is the first demon to appear on the show.
- According to Sister Jude, Satan uses the purity of Sister Mary Eunice as a shield to protect himself from the holy power of sacred objects, including the Bible, Crucifix, and Holy Water.
- Actress Lily Rabe, who portaryed Sister Mary Eunice, stated that the Devil's behaviors change depending on his host. 
- Satan is the second immortal character to be introduced in accordance with each season; The first was his cousin, Shachath. The third, fourth and fifth immortal characters introduced were Marie Laveau, Delphine LaLaurie and Bastien (all of which later had their immortality revoked). The sixth was Papa Legba. The "seventh" were a handful of vampires. The eighth was The Addiction Demon. The ninth was Scáthach.
- Satan is heavily implied to be male, or at least a masculine being, as Michael Langdon referred to him as "Father" and Vivien Harmon stated that Satan is Michael's "true father". Despite his gender identity, the Devil seems to engage in sexual activities with both men and women, indicating he is bisexual (if the term can be applied an otherwordly entity). He possesses an extremely hedonistic and sexually voracious personality, taking great pleasure in the suffering of others via an overtly sexual manner, while simultaneously encouraging people to follow a lifestyle of overindulgence and base urges.
- As relayed by Papa Legba, Satan is hated by other powerful supernatural forces, such as the Voodoo Loas and the Old Gods. This is due to the fact that if the Devil were to successfully bring about the end of the world via the Anti-Christ - thereby ending mankind - other deities would lose their human worshippers and, thus, become irrelevent.
Evidence of the Character's Identity
A hotly debated topic is whether the entity that took over Mary Eunice was, in fact, Satan himself, or a lesser demon. However, in the show, the character was frequently referred to as "the Devil" (and even "Satan" on a couple occasions), including when he talked about himself. Show creator Ryan Murphy has stated, when describing the character, "This is a much more sly devil, a more sly Satan. I like the idea of the devil as ambitious.". Furthermore, Murphy has stated, regarding Eunice/Satan's end, "We didn't want she dies and the devil goes into someone else. I liked the idea that the only thing more powerful than the devil is the Angel of Death, which is actually the cousin of the devil." 
- If the Devil is actually Satan himself, then he is directly linked to Murder House. According to Billie Dean Howard, he is associated with Michael Langdon, the Anti-Christ fated to bring about the end of the world. Given that each season has been confirmed to exist in the same shared universe, this theory is highly likely, implying that the Devil may return in a future season. This is hinted throughout Apocalypse, until directly confirmed by the events of Return to Murder House.
- Sister Mary Eunice was killed after being thrown from the top of a spiral staircase by Timothy Howard, during which the camera panned from a birds-eye view as she fell to her death. In Welcome to Briarcliff, Sister Jude refers to this staircase as her "Stairway to Heaven"; consequently, Mary Eunice's descent downward may be symbolic of the Devil's fall from power or Lucifer's fall from grace. Interestingly, several seasons have featured similar spiral staircases, and all have been displayed from a birds-eye view at least once or more in their retrospective seasons. All of these staircases exist in places known for having dark and sordid histories – the Murder House, the Roanoke House, Briarcliff Manor, Mott Manor, and Outpost 3 – implying a possible connection to the Devil. This may be further supported via the as-of-yet-explained demonic murals found painted on the walls of the Murder House , which have a striking resemblance to those painted by Constance Langdon - the adoptive grandmother of the Anti-Christ.
- ↑ Mentioned by Leah
- ↑ Mentioned by Billie Dean Howard
- ↑ Voice
- ↑ Michael Langdon speaks to him via a Satanic ritual, asking him for guidance on how to dispose of the witches
- ↑ Mentioned by Michael Langdon, Miriam Mead and Cordelia Goode
- ↑ Mentioned by Miriam Mead
- ↑ Mentioned by Papa Legba
- ↑ Mentioned by Michael Langdon and Madelyn
- ↑ Mentioned by Jeff, Mutt, and Cordelia
- ↑ Mentioned by Michael Langdon, Cordelia Goode, Myrtle Snow and Mallory
- ↑ Episode: Tricks and Treats
- ↑ Episode: Nor'easter
- ↑ Episode: Dark Cousin
- ↑ Episode: The Coat Hanger
- ↑ Episode: The Name Game
- ↑ Episode: Birth
- ↑ Episode: Pilot
- ↑ Episode: Return to Murder House
- ↑ Christina Radish, Lily Rabe Talks American Horror Story: Asylum, Her Character’s Fate, Favorite Storylines, and More, Collider.com, Jan 16, 2013
- ↑ EW 'American Horror Story': Ryan Murphy on fate of...
- ↑ EW American Horror Story': Ryan Murphy teases season 3
- ↑ Murder House (location)/Wall Murals
- ↑ Episode: Spooky Little Girl
- ↑ Episode: Afterbirth