Personality & Appearance
At least while at Briarcliff, the Demon lacked a true form, and only existed in a host. While its behavior while in control of Jed Potter was monstrous and uninhibited, it initially stayed very low key under the guise of Sister Mary Eunice, subtly manipulating and mentally torturing Briarcliff's inhabitants (especially Sister Jude with her dark past, and Dr. Arden with his previous life as a Nazi scientist). However, as time went on, the Demon revealed itself to Jude (who it had previously encountered during Jed's exorcism) and Dr. Arden (who desperately followed the Demon in hope of seeing at least a glimmer of Mary Eunice, whose innocence he cherished). After Jude was removed as head of Briarcliff and incarcerated within, Mary Eunice grew closer to Monsignor Timothy Howard, promising to help elevate him through the ranks and make him Pope. The Demon's master plan was thus set in motion: a take over of the Catholic Church. It had the power to possess people, make their eyes glow demonically, as well as reading the minds of people and telekinetically moving objects. The demon often enjoyed reading a person's mind to see their secrets, such as Sister Jude's alcholic past and the "murder" she commited to its own amusement. The demon thrived solely through possession and if its host died, it died along with him, making the host both its strength and its 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.
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 the 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/