Personality & Appearance
At least while on Earth, the Demon lacked a true form, and only existed in a host. While in control of Jed Potter, its 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, it 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. It 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 Demon to blackmail him into carrying out its commands. However, as time went on, the Demon eventually chose to reveal itself to Jude, who it had previously encountered during Jed's exorcism, and Dr. Arden, who desperately followed the Demon 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, the Demon grew closer to Monsignor Timothy Howard, promising to help elevate him through the ranks and make him Pope. Its master plan was thus set in motion: a take over of the Catholic Church.
Despite its sly and ambitious nature, however, the Demon 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 off of the rules of man and religion, and should therefore be able to make their own destinies. Despite this, the kind of behavior it encouraged in others, such as Jenny Reynolds, was violent and disturbing, and would thereby contribute to its overall plan of bringing destruction in the faith of God and the downfall of mankind.
In addition to its mind-breaking influence, the Demon 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 its mind and reading the minds of all those around it, where it took great joy in seeing other's darkest secrets and manipulating them for its own gain and amusement. Unfortunately, because the demon thrived solely through possession, it could not escape the kiss of Shachath, the Angel of Death, as failure to escape its previous host's body in time would result in it dying along with them, making the host both its strength and ultimate weakness.
After initially getting into Briarcliff through Jed, the Demon took over young, innocent Sister Mary Eunice, and slowly used malice and manipulation to get Jude fired from Briarcliff. Acting as Eunice, it 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 acts of turning "away from God". Still, despite the public face as sweet, innovative Mary Eunice, the Demon was commiting 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 Demon used this murder to reveal himself to Jude).
At one point, the Demon confronted Shachath, the Angel of Death, demanding she leave Briarcliff after the Angel was "called" by a dying inmate. Shacath retorted that she had been invited, unlike the Demon, 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 Demon, and momentarily, Mary Eunice took control of her own body again, sobbing and pleading to Shachath to free her. The Demon quickly resumed control; before leaving, Shachath warned it they would meet again.
Soon, the Monsignor learned of Mary Eunice's possesion after being told about it by Shachath, and he attempted to cast the Demon out of Mary Eunice. However, the Monsignor was overpowered and violated by the Demon, taking his virginity and that of Mary Eunice. 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, the Demon confronted Timothy and repeated its 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 the Demon, 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 the Demon, 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. Shacath 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.
The Demon is a powerful spiritual entity with a variety of supernatural powers.
- Immortality: The Demon is immortal and cannot be physically killed. It is not subject to old age or disease.
- Possession: The Demon can possess humans, taking control of a host's mind and body to manipulate for its own purposes.
- Telekinesis: The Demon can move objects and people without touching them.
- Clairvoyance: The Demon can enter into the minds of others, read their thoughts, and access their memories, granting it an inherent knowledge of others' past sins, regrets, and moral choices.
- Concilium: The Demon can influence the minds of others through seduction and persuasion.
- Transmutation (presumably): The Demon can move instantaneously from one location to another.
- To Dr. Arthur 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. Arthur Arden: "You really think you're in charge? You touch me again, you'll die."
- To Jenny: : "Poor Mary Eunice! Blah!"
- To Dr. Arthur Arden: "Did you celebrate Christmas in your Nazi household?"
- To 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. Arthur Arden: "Rubies are the most glamorous of all. You couldn't have found them in this dreary town. Are they family heirlooms?"
- To Dr. Arthur Arden: "Jesus Christ... You're being pitiful Arthur."
- To Jenny: "I know everything. I'm the Devil."
- This is the first demon to appear on the show.
- Though the Demon had previously avoided death by taking shelter in another host, statements by the show's creator Ryan Murphy have confirmed that it has passed on alongside Mary Eunice (see below).
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. Furthermore, 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." 
- Tricks and Treats - Jed Potter is exorcised at Briarcliff Manor by Timothy Howard and Father Malachi and upon his death the demon apparently finds a new home with Sr. Mary Eunice.
- Nor'easter - Sr. Mary Eunice's formerly chaste and obedient demeanor is replaced by behavior that ranges from mischievous to malevolent; wearing makeup, delighting in Hollywood depictions of Christian slaughter, seduction, psychological torture and murder.
- I Am Anne Frank: Part 2 - Sr. Mary Eunice continues her uncharacteristic behavior; shielding Dr. Arden from being exposed as a sadistic sociopath, plotting the downfall of Sr. Jude and intervening in the sterilization of Grace (not in her favor).
- The Origins of Monstrosity - the Demon uses the guise of Mary Eunice to encourage Jenny Reynolds to nurture her destructive, homicidal impulses. Later, in response to Sr. Jude's dismissal from Briarcliff, the Demon dresses in Jude's red lingerie and dances mockingly in front of an image of the crucified Christ. When Sam Goodman calls, she kills him and takes his evidence of Arden's misdeeds in order to further manipulate him.
- Dark Cousin - the Demon is briefly confronted by Shacath, the Angel of Death, who promises they will meet again.
- ↑ http://insidetv.ew.com/2012/11/01/american-horror-story-ryan-murphy-chloe-sevigny/
- ↑ http://insidetv.ew.com/2013/01/03/american-horror-story-ryan-murphy-the-name-game-exclusive/