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According to My Roanoke Nightmare, Scáthach was once an English woman who was descended from the Druids and their Roman conquerors. A worshiper of old gods, she left for the British colonies 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. The soldiers imprisoned her and sentenced 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, Scathach came across the exiled former leader of the Roanoke colony, Tomasin White, who she spared from 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, Tomasin 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.
Over the hundreds of years since, many 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
Scáthach is often seen wearing rags and having dirt and blood on her face and body. She has long and thick russet hair and an animal skull on the top of her head. She usually carries bloody human or animal remains. She is commonly heard cackling in a childlike yet sinister voice which echoes in the woods, and screeches like a vulture when others invoke her wrath.
She 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. Scathach 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, Scathach 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, Scathach disappears without a trace.
After Shelby and Matt accept the help of Cricket, the psychic medium calls out the attention of Scathach, demanding information on the paranormal events happening in exchange for offering her Matt. Appearing suddenly from the night fog itself, Scathach blows a dark enchanted powder into his eyes, blinding him as she intends to kill him with a ritualistic dagger. After hearing his offer to give her Matt, however, she takes him many years back in time, agreeing to show him what he desires. 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 Scathach'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 Scathach awaits. Upon this being discovered by Shelby, Scathach 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 Scathach that broke apart his and Shelby's marriage. Shelby reacts by violently bludgeoning him in the head with a crowbar.
On the final night, Scathach 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.
- Telekinesis: The power to move objects with one's mind. Scáthach was able to telekinetically remove a cage from the Butcher's head.
- Transmutation: The power to instantaneously move from one location to another. Scáthach magically disappeared whilst being chased by Shelby Miller.
- Concilium: The power to control the mind. Scáthach was able to compel Matt Miller into having sex with her, and erase his memories of the event.
- Eternal Servitude: The power to control others by stealing their souls, performed via ceremony. Scáthach used this power to control the Butcher and Lee Harris, forcing them to eat a boar heart that epitomized the dispension 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 power 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.
- Pyrokinesis: The power to manipulate fire with one's mind. As Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- Divination: The power to gain knowledge through indirect means. As Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- Vitalum Vitalis: The power to balance the scales of life and death. As Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- Descensum: The power to descend into the afterlife. As Supreme, it is implied that Scáthach possesses this and all other Seven Wonders.
- This is the second American Horror Story character portrayed by Lady Gaga. The first being the Countess in Hotel.
- Scáthach is the seventh immortal character to be introduced; first were Marie Laveau and Delphine LaLaurie (both of which later had their immortality revoked). The third immortal introduced was Papa Legba, a deity, and the the fourth, fifth, and sixth immortal characters introduced were vampires: The Countess, Donovan, and Ramona Royale.
- 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".
- 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 hair, 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 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.
- The gods she worships 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.
- Ryan Murphy has confirmed that Scáthach is the original Supreme, pre-dating the Salem witches by centuries. 
- It's unknown who portrayed the real Scáthach, but it was rumored to be Alexandra Daddario.
- 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 Episode: Chapter 3
- ↑ Episode: Chapter 4
- ↑ Ryan Murphy Confirms Lady Gaga’s Character Is The Original ‘American Horror Story’ Supreme.