Caster is a servant from Fate/stay night. She became a Honored guest of the Ryudou Family and was able to stay at their temple unitl preparations for her marriage have been finalized. She was really using the temple to revive the Holy Grail.
In her lifetime, Caster is better known as Medea, an archetype for witches from Greek mythology. She is a princess, the daughter King Aeëtes of Colchis and a niece of Circe, both of whom were distinguished spellcasters. Under the tutelage of her father, Medea herself becomes a skilled sorceress, living a quiet, peaceful life until the arrival of Jason and the Argonauts.
Jason, the legitimate heir to the throne of Iolcos in Thessaly, sails to Colchis on a quest for the Golden Fleece. His uncle, King Pelias, imprisons Jason's father and assumes the crown. Fearful of being deposed, Pelias consults an Oracle who tells him to shun any approaching man wearing one sandal. Fatefully, this man would be Jason, who loses a sandal escorting a disguised Hera across the Anauros river. To avoid a disgraceful murder, and upon Jason's suggestion, Pelias requests Jason obtain the Golden Fleece, which he considers to be a suicidal mission.
Upon his arrival in Colchis, Jason reaches an agreement with King Aeëtes, whereby he can take the Golden Fleece if he proves capable of accomplishing three impossible tasks. As a favor to Hera, Aphrodite orders her son Eros to beguile Medea into falling madly in love with Jason. With her aid, Jason is able to acquire the Golden Fleece, forcing the Argonauts to flee Colchis. Aeëtes, not expecting Jason to actually win the prize, values the skin enough to give chase. As a distraction, Medea kills her brother, Apsyrtus and scatters his diced flesh into the ocean brine. Aeëtes, horrified at the murder, delays his vessel to retrieve the pieces of his son.
Fatigued from the taxing voyage, the Argonauts seek temporary asylum in Crete, but they are waylaid by Talos, a giant bronze automaton originally gifted to Europa by Zeus. As sentinel of the isle, Talos heaves large boulders at the Argo, tossing the ship about like a cork. Medea hypnotizes him with her sorcery, and in a bout of insanity, Talos removes a thick bronze nail from his heel. Without this nail, the ichor from within Talos' body gushes out, poisoning and killing him. With Talos' death, the Argo safetly docks on the isle.
At the end of their extensive journey, the Argonauts return to Iolcos, but Pelias refuses to yield his throne to Jason. Hera, angry with Pelias, manipulates Medea into murdering him; Medea approaches Pelias' daughters and offers to restore their father's youth and vitality. As a demonstration, Medea asks that Jason's father, the ailing Aeson, be summoned from Pelias' dungeon; when he arrives, Medea slits his throat and submerges his lifeless body in a hydria of boiling water. After Medea adds several magical herbs, Aeson emerges alive and rejuvenated. Captivated by Medea's sorcery, Pelias' daughters butcher their father and dump his body parts into the frothing cauldron; however, Medea withholds her herbs and Pelias remained dead.
Acastus, a fellow Argonaut and son of Pelias, is outraged by the murder of his father; Jason and Medea are driven from Iolcos and claim sanctuary in Corinth. While there, King Creon urges Jason to forsake Medea and marry his daughter, Glauce; although Medea bears Jason two children, he concedes wedding Glauce to strengthen his political connections. Stunned and aching from Jason's betrayal, Medea presents Glauce with bewitched clothing as wedding gifts. When Glauce dons these garments, they cling to her body and spontaneously combust; she is soon consumed by the flames, as is Creon, who embraces his dying daughter. To spite Jason, Medea murders her own children and takes their dead bodies with her as she flees Corinth atop the chariot of her grandfather, Hêlios.
Medea makes her way to Athens, where she marries King Aegeus and gives birth to a son, Medus. The two live a blissful and serene life until Theseus, a son of both Aegeus and Aethra, arrives in Athens. Medea immediately identifies who he is; wishing to ensure Medus' inheritance of the Athenian throne, she plots to kill Theseus. Playing on Aegeus' suspicions of the newcomer, she orders Theseus to prove his valor by capturing the Marathonian Bull. After he succeeds, Medea offers him a skyphos of poisoned wine. Before Theseus can sip the poison, Aegeus recognizes the equipment he is wearing and realizes Theseus is his son. Aegeus knocks the cup out of Medea's hand; the reunited father and son embrace.
Upset over the attempted murder, Aegeus angrily forces Medea and Medus out of Athens; they return to Colchis, where they discover Aeëtes has been killed by his brother, Perses. As a precaution to protect his seat on the throne, Perses imprisons Medus; furious, Medea impersonates a priestess and calls down a plague upon Colchis. When Perses seeks his priestess' council, Medea advises that she be sacrificed as a tribute to the gods. Perses concedes, but amidst the sacrificial ceremony, Medea grabs the ritual knife and uses it to kill her uncle. Medus is freed and becomes the new King; he later annexes a nearby country and renames it in honor of his mother.
Typically, Heroic Spirits of an evil nature like Caster are not eligible for participation in the Fuyuki Holy Grail Wars; however, after the Holy Grail was tainted in the third war, most of the restrictions against such spirits were lifted. Caster's first Master was a member of the Mage's Association, but Caster killed him in retribution for his cruelty and abuse. On the verge of her spiritual body dissipating, she stumbled upon Kuzuki Souichirou at the foot of the Ryuudouji Temple. Caster begged this stranger for his help; Souichirou obliged without hesitation, forming a new contract with her. He then brought Caster into the temple and attended to her recovery. Caster quickly fell in love with Souichirou, for he was the first man to show her genuine kindness, dedication and loyalty. Her primary objective shifted from obtaining the Holy Grail for herself to simply preserving their short time together. To achieve this end, she has been siphoning Mana from the populace of Fuyuki City and has been fortifying her position on Mount Enzou.
Caster is killed in every scenario of Fate/stay night. In Fate, her stronghold in the Ryūdouji Temple is briefly assaulted by Saber, though her own Servant, Assassin, manages to hold her back. She is later torn apart by Gilgamesh 's Gate of Babylon while ambushing Emiya Shiro and Saber. In Unlimited Blade Works, she manages to stab Saber with Rule Breaker and negate her contract with Shirou; Caster then abducts Saber and tortures her, hoping to provoke Saber into becoming her Servant. While attempting to kill Emiya and Tohsaka Rin in the basement of the Kotomine Church, she dies protecting her Master from Archer's hailstorm of blades. Caster is killed early on in Heaven's Feel by Saber, but Matou Zouken uses one of his worm familiars to maintain her body as a puppet.
In Fate/hollow ataraxia, Caster goes about as a normal woman, normally hanging around the mall. She is the first person to discover the four-day time loop (other than the player). She also fights the dog-shadows using anti-army spells, often coming close to killing her allies.
Fate/Stay Night: Unlimited Blade Works
An alternate retelling of Fate/Stay Night based off of the original visual novel's scenario by the same name. It primarily focuses on Shirou, Archer, and ...