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According to My Roanoke Nightmare, Scáthach is an English woman descended from the Druids and their Roman conquerors, who left for the British colonies sometime during the 14th century as a stowaway on a voyage that was plagued with death. Upon being discovered at landfall, the British soldiers blamed Scáthach's female presence for enraging the sea gods and imprisoned her, sentencing her to be burned at the stake as a witch. Scáthach answered the call of more ancient and bloodthirsty gods and massacred the soldiers, thereby incriminating the Native Americans and allowing her to escape into the wild.
In the late 1500s, Scáthach came across the exiled former leader of the Roanoke colony, Thomasin White, whose life she spared from being devoured by a wild boar, in exchange for her soul. After White returned to Roanoke and violently regained control of the colony, the colonists were subjugated to follow in Scáthach's religious ways. The colony migrated to a location in North Carolina, one hundred miles away. White's own son, Ambrose, defied his mother and called on the colonists to return to Christ. As such, Thomasin personally killed the entire colony, so that their souls would belong to Scáthach. As a final component of the deal, White let Scáthach take her life, for eternal posthumous servitude.
In the centuries since, many have encountered Scáthach and her ghostly Roanoke followers. While most would die under White's blade, Scáthach had a strong sexual appetite that she loved to have sated by hapless men.
Personality and Appearance
In My Roanoke Nightmare, Scáthach is portrayed as being much more human-like in both appearance and personality, albeit exceptionally cunning and mischievous, with a hidden purpose none can yet comprehend.
After overhearing a caged, starving Thomasin's pleas for help in the name of the Christian God, Scáthach marks her first appearance by interruppting the mortal's cries via materializing behind her in the darkness, violently slaughtering the wild boar which nearly took her life. Spattered in blood, she can be seen smiling to herself when she offers Thomasin the still-beating heart of the boar, whispering for Thomasin to hand over her soul in exchange for saving her; an act which Thomasin oddly complies with, giving the implication that Scáthach could have already been influencing her before making her presence known.
When the other Roanoke colonists grow distrustful of Scáthach, the witch immediately engages Thomasin in a private conversation, convincing her that she need only turn against her fellow men and women to retain her dominance over them. Her black teeth curl in a crooked grin when she describes the power of the October Blood Moon and a terrible curse which could be cast to enslave the colonists to Thomasin forever. Consequently, Scáthach would appear to have sinister motives, using dark magic to persuade and utilize others into carrying out whatever she commands of them, all while appearing inconspicuously quiet.
She may have reservations regarding some of her past actions, however; when psychic-medium Cricket Marlowe confronts her, hoping to better understand the murderous sacrifices of the Roanoke Colony, Scáthach consents to showing him visions of the past. One of these past memories consists of Priscilla's death by the Butcher's hand, and Scáthach appears to look exceptionally solemn at having to re-watch the child's sacrifice. This may imply that she feels regret over the innocent lives lost to appease the wrath of the old gods. Whether or not this is true, or merely a fabrication on My Roanoke Nightmare's part for entertainment value, is unknown.
In reality, it would appear more likely that Scáthach's character has been misrepresented on television, as her actions and behavior during the events of Return to Roanoke come across as solely malevolent, with no hint of remorse for what can be seen as virulent, sexually voracious, and animalistic behavior. Like the spirits of Roanoke under her command, she also appears to look much less human in appearance than her fictionalized counterpart, displaying characteristics more akin to having diseased or rotting green flesh. She is dressed in tattered rags, her face and body soiled with dirt and blood, while her long, thick russet hair is adorned with an animal skull. She is usually carrying bloody human or animal remains, and is commonly heard cackling in a childlike yet sinister voice which echoes in the woods, screeching like a vulture when others invoke her wrath.
Scáthach participates in the sacrifices the colony holds.
Via psychic medium Cricket Marlowe, Lee and Shelby are able to communicate with The Butcher, hopeful that in doing so they will be able to find Flora. Scáthach uses this as an opportunity for her advantage, luring Matt away from his family by enchanting his mind to do as she wills, influencing him to have sex with her in front of her subordinates. 
When Elias shows up at the Roanoke House, he offers to help Matt and Shelby find Flora by taking them to a portion of the woods where Priscilla likes to play. On the way, Scáthach appears and beckons Matt to come to her, but this becomes interrupted by a furious Shelby, who chases her into the forest. Before Shelby can catch up with her, Scáthach disappears without a trace.
After Shelby and Matt accept the help of Cricket, the psychic medium calls out the attention of Scáthach, demanding information on the paranormal events happening to the Miller family. Appearing suddenly from the night fog, Scáthach blows a dark enchanted powder into his eyes, blinding him as she intends to kill him with a ritualistic dagger. Frightened by the witch, Cricket offers to give her Matt in exchange for sparing his life, and the witch consents, agreeing to show him what he desires by taking him many years back in time. Under her spell, Cricket sees the beginning of the Roanoke colony under her rule, when the ancient murder of children took place by the hands of The Butcher, who sacrificed them in the name of the old gods to appease their wrath and prosper in the colony's chosen land.
Later, Matt hears frightening noises outside and hesitantly leaves the house to investigate, only to once again fall under the sway of Scáthach's enchantments. Despite trying his hardest to defend himself against his growing urges for her, he is incapable of denying her mind-bending influence on him and falls to her will, once again offering up his body to her. Through her sorcery, she unveils her backstory to him via bizarre, dream-like hallucinations during their encounter.
In the nights of Return to Roanoke: 3 Days in Hell, Matt begins to sleepwalk to the basement, where Scáthach awaits. Upon this being discovered by Shelby, Scáthach emits an angry inhuman screech at the mortal before vanishing into the darkness, leaving Matt to confess that he only agreed to participate in the show because of his love for the witch, admitting that it was this undying devotion for Scáthach that broke apart his and Shelby's marriage. Shelby reacts by violently bludgeoning him in the head with a crowbar. 
On the final night, Scáthach offers Lee a freshly-snatched boar heart, reminiscent of what she did with The Butcher centuries ago, which results in Lee becoming just as evil and bloodthirsty as her predecessor. 
- Immortality - The ability to live forever in eternal life, immune to all aging, disease, and death.
- Telekinesis - One of the Seven Wonders, the magical ability to manipulate objects with one's mind. Scáthach was able to telekinetically remove a cage from the Butcher's head.
- Transmutation - One of the Seven Wonders, the magical ability to move instantaneously from one location to another. This power is more commonly known as Teleportation. Scáthach magically disappeared whilst being chased by Shelby Miller, and later appeared suddenly from the night fog to threaten Cricket.
- Concilium - One of the Seven Wonders, the magical ability to bend the will of another individual and force them to do whatever the user desires. This power is more commonly known as Mind Control. Scáthach was able to compel Matt Miller into having sex with her and erase his memories of the event.
- Pyrokinesis - One of the Seven Wonders, the magical ability to create, control, and manipulate fire with one's mind. As a Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- Divination - One of the Seven Wonders, the magical ability to gain knowledge through indirect means. As Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- Vitalum Vitalis - One of the Seven Wonders, the magical ability to balance the scales of life and death. As a Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- Descensum - One of the Seven Wonders, the magical ability to descend into the netherworlds of the afterlife. This power is also known as Astral Projection. As a Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- Enhanced Agility - The ability to perform motions, or movements, at a speed and effortlessness that surpasses that of a normal member of the user's species, possessed of enhanced speed, balance, endurance, flexibility, and reflexes.
- Eternal Servitude - The magical ability to control others by stealing their souls, performed via a ceremony. Scáthach used this power to control both the Butcher and Lee Harris, forcing them to eat a boar heart that epitomized the dispensation of their free will. Any injuries inflicted upon the recipients prior to them consuming the heart appear to be healed after this, as Lee seemed invincible despite her scarred leg. Thus far, Scáthach is the only witch within the series that has displayed this ability.
- Illusion Manipulation - The magical ability to 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. Scáthach used this power on both Matt Miller and Cricket Marlowe; first by showing Cricket visions of the Roanoke colony's sacrificial beginning, and then on Matt when revealing visions of her past.
- This is the second American Horror Story character portrayed by Lady Gaga. The first being the Countess in Hotel.
- Lady Gaga recorded her role as Scáthach as part of behind-the-scenes footage for her documentary "Gaga: Five Foot Two", in which her scenes were filmed during the recording of her fifth album. 
- For unknown reasons, Scáthach has displayed an interest in both of the Miller siblings - Matt as her consort and Lee to become a new, living Butcher. The reasoning behind this remains a mystery and has yet to be revealed.
- Scáthach is the eighth immortal character to be introduced in accordance of each season; the first and second immortal characters introduced were Shachath, the Angel of Death, and her cousin, The Devil. The third and fourth immortal characters introduced were Marie Laveau and Delphine LaLaurie (both of which later had their immortality revoked). The fifth was Papa Legba, The Voodoo Loa, "sixth" were a handful of vampires, and seventh was The Addiction Demon.
- Scáthach is arguably one of the oldest-known characters in the AHS universe, alongside The Devil, The Addiction Demon, The Angel of Death, Papa Legba, and the old gods.
- Scáthach's name comes from a legendary warrior woman figure in the Ulster Cycle of Irish mythology.
- In Irish, Scáthach means "shady", or more literally "shadowy".
- Scáthach may be inspired by Arduinna, a Celtic goddess of the moon, hunting, and forests, whose sacred animal is the wild boar.
- Unlike the witches seen in Coven, who look like normal human beings, Scáthach appears to be more in line with the archetypal hag or crone version of witches portrayed in fairy tales and folklore, possessing sharp yellow fangs, mossy greenish skin pervaded with black veins, long scraggly russet hair topped with an animal skull (which she uses as a ritualistic headdress), and unnatural, mismatched eyes. She has the power to steal her victims' souls, cackles wickedly in the forest, and vocalizes a frightening, unearthly shriek when angry. She also refers to the Roanoke colony as "children" - a possible reference to Hansel and Gretel.
- Her hag-like appearance is most likely due to the dark and primal magic she uses, and presumably a direct result from her pact with the old gods. Her appearance may also be due to the fact that she lives in the forest, most likely using whatever she can find to dress herself, evident in the animal bone she wears on her head and the leather she wears, most likely from animals she has killed herself.
- Her greenish skin is similar to the Wicked Witch of the West from the Wizard of Oz 1939 film.
- In her true form, Scáthach appears to have one glass-like, milky white-colored eye. It is unknown if this is because she is partly blind, wears an artificial eye, or if this eye possesses its own magical qualities. Regardless, both eyes appear to be equally bloodshot at their scleras.
- Ryan Murphy has confirmed that Scáthach is the original Supreme, pre-dating the Salem witches by centuries. 
- Opposed to being born a witch, Scáthach was a normal human being who became one through the worship of ancient deities. These aforementioned gods may hail from both ancient Roman and Druidic practices, as she is said to descend from both the Druids and the Romans. Her magical powers may exist as a result of the human sacrifices she demands the Roanoke colony to carry out; consequently, it is possible that if these sacrifices were to end, she would lose her magic. However, it is also possible that the old gods taught her magic and, as a means of honoring them, she continues to perform sacrifices in their name.
- In "Bitchcraft", the pilot of Coven, Delphine LaLaurie mentions vengeful gods of Greek mythology. It is possible that Scáthach worships several of these gods.
- Due to the Roanoke Colony committing human sacrifices in the name of the old gods, Scáthach has been granted endless life, thereby allowing her to surpass the average 30-50 year lifespan of other Supreme witches. The relationship between Papa Legba and Marie Laveau was very similar to this, as Legba kept Marie immortal in exchange for annual human sacrifices.
- To The Butcher: "Surrender thy soul... to me."
- To Cricket: "Losing your sight is the least of your problems."
- To The Butcher: "They are all children in need of discipline. Bend them to thy will. Shed their blood. Release the power of the Blood Moon. Thou shalt have their souls enslaved to thee. Thy servants for eternity."
- To Matt Miller: "My treasure."
- To Matt Miller: "Debts must be paid."
- Scáthach: "It was decided I would be burned at the stake as a witch, but there were other gods demanding blood. More ancient and thirsty gods."
- Matt Miller: "The Old Magic and New World created something new. Something original."
- ↑ 1.0 1.1 Ryan Murphy Confirms Lady Gaga’s Character Is The Original ‘American Horror Story’ Supreme.
- ↑ 2.0 2.1 Episode: Chapter 3
- ↑ Episode: Chapter 4
- ↑ Episode: Chapter 7
- ↑ Episode: Chapter 9