Personality and 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.
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.
Largely cold, emotionless, and sly in disposition, the demon rarely smiled or laughed save for taunting others and mocking the fates of those harmed by it. It had very little regard for human life, and had no qualms in murdering and manipulating various mortals in cold blood. It found great satisfaction in humankind's many flaws, and frequently reminded its victims of this in various violent and psychologically harmful ways. It 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. It seemed very comfortable in Sister Mary Eunice's skin, even in private, and used its hold over her body to take away her virginity without her consent, dominating what little bit of her soul still remained in her.
Though sly and ambitious in nature, the demon was not without its redeeming qualities. It 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 it exhibited toward adults, it appeared to be very docile around children, calmly and honestly exposing its true identity to Jenny Reynolds and smiling down at her in a friendly gesture. Despite this, the kind of behavior it encouraged in Jenny, as well as other individuals, such as Leigh Emerson, 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 being acts of turning "away from God". Still, despite the public face as sweet, innovative Mary Eunice, the Demon 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 Demon used this murder to reveal himself to Jude). 
At one point, the Demon 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 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 possession 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. 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 hell and presumably throwing the Demon's soul back into Hell. 
The Demon was 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 people, taking control of a host's mind and body to manipulate for its own purposes.
- Telekinesis: The Demon can move objects and people with its mind.
- 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: The Demon can move instantaneously from one location to another.
- 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 Jenny: "I know everything. I'm the Devil."
- To Jenny: "She deserved it that Josie, she was just a phony little shit!"
- This is the first demon to appear on the show.
- According to Sister Jude, the Demon uses the purity of Sister Marie Eunice as a shield to protect itself from the holy power of sacred objects, including the Bible, Crucifix, and Holy Water.
- Actress Lily Rabe, whose character Sister Mary Eunice was possessed by the Devil, stated that the Demon's behaviors change depending on the host. 
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 Demon 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 Demon may return in a future season.
- 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 Demon'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, and Mott Manor – implying a possible connection to the Demon. 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.
- ↑ Episode: Tricks and Treats
- ↑ Episode: Nor'easter
- ↑ Episode: Dark Cousin
- ↑ Episode: The Coat Hanger
- ↑ Episode: The Name Game
- ↑ 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