Breaker Gorgon: Self Seal - Dark Temple (自己封印・暗黒神殿, Jiko Fūin - Ankoku Shinden?) is the blindfold of Medusa's, a type of powerful Bounded Field Noble Phantasm on the scale of another World. It was a complement to the Temple of Blood that "enveloped the World" on the Shapeless Isle, having "sealed the World" instead. It only requires a steady supply of energy from the user to affect a target, unlike Mystic Eyes that requires a direct line of sight.
It has the ability to "seal all magical properties", and if used on others, it will imprison the consciousness of the enemy within her own mind; it can even obstruct the invocations of abilities such as magic, Skills, or Noble Phantasms, before removing any abilities they possessed in the outside World once inside. Despite this ability Rider didn't use Breaker Gorgon offensively in the entirety of the Fifth Holy Grail War.
Those trapped within are said to see a nightmare mixed with both delight and taboo. When Medusa becomes inadvertently trapped by Shirou reversing it on her, both the abilities of her eyes and her superhuman strength are removed. It is manifested with a magic circle in the same pattern used to summon the Pegasus, and it dyes the affected target's eyes crimson.
Mystic Eye Killers
This Noble Phantasm is used by Medusa on herself mainly as self-imposed Mystic Eye Killers, sealing her excessively powerful Mystic Eyes of Petrification within the Bounded Field. It completely seals her sight, and it is generally manifested as a physical blindfold. Sealing her eyes also means completely sealing her eyesight, so she instead perceives the world through senses such as hearing, touch, smell, and magic-detection that allow her to acquire more precise information than sight. When she uses it in a makeshift manner on herself through a mirror to seal her magical energy to suppress the effects of her eyes without manifesting the blindfold, she displays a helplessness after accidentally sealing her eyesight as well, but displays the ability to keep her eyesight while sealing her eyes afterward.
Medusa is able to replace its everyday usage with a pair of Mystic Eye Killer glasses at the end of the Heaven's Feel route and contacts in Fate/hollow ataraxia. They do not act on all magical properties like Breaker Gorgon, so they only seal the effects of her Mystic Eyes. Once she releases it, her eyes can be used continuously to affect her enemies.
When Medusa used it as the Gorgon on Perseus, it would have been able to affect him without any recourse or need for the Gorgon to actually focus on the battle, but Kibisis instead caused it to be reflected back upon her like a mirror. It caused her to suffer a "nightmare beyond all nightmares" as Medusa, a reflection of when she had self-awareness. Completely immobilized, she was struck down by Harpe. Discussing the "nightmare" later, a dream where she was harassed by Stheno and Euryale, she states that it was not an evil nightmare that paralyzed her with excruciating pain, but rather the joy of seeing those she thought she could no longer meet. The incident caused her to have a hatred of mirrors, which she believes might be from the fear of reminiscing about those days.
Medusa can use it on a target to capture them for the purpose of taking in blood and semen under the false pretense of a lewd dream. Taken within it during Fate/hollow ataraxia and shown a sexual encounter with Medusa and Sakura, Shirou senses it has an ambiguous nature with no meaning in trying to determine the time or location. The senses felt by his body are ambiguous, only feeling "all-encompassing sweetness of flesh" of Sakura and Medusa. It removes all painful memories and the sense of reality as his memories and normal feelings are washed away. He says the him within it is not real, but rather an illusion shaped in the image of his and Sakura's souls.
Seeing the illusion of Sakura causes a brief sense of realization that the room, and the World, is a mistake. Although he can remember being drawn in, he had not expected to see Sakura. Due to the difference in Sakura's personality, her "true self" helping Shirou better embrace her according to Medusa, he notices that it is an illusion. Somehow reading the memory of Medusa's nightmare as the Gorgon allows him to break the barrier, easily preparing a projection of Kibisis due to it being inside of her spirit rather than reality. He is able to reverse the barrier "like a reproduction of mythology", bringing her in within the "dream" as well. "Sakura" explains that the illusion of Sakura is really just a means to mask Medusa's own feelings for Shirou as she takes in his blood. He has no recollection of it afterwards.
Using the illusion in Heaven's Feel, she has a sexual encounter with Shirou under the guise of Rin Tohsaka, the only difference being her real eyes showing on occasion. In Realta Nua, Shirou is instead compelled to follow "Rin" throughout the town as she enters their school. He comes upon her preying on Ayako Mitsuzuri for her blood, and he is unable to resist as she preys upon him as well, briefly showing her true eyes. The "World" is notably dyed in red in both scenarios.
 Fate/complete material III: World Material - The Servants of the Fifth Holy Grail War: Rider, p.028-029 [T]
Master: Matou Sakura
Alignment: Chaotic Good
Noble Phantasm: A+
Medusa was a frightening female monster in Greek mythology. She had a nest of poisonous snakes for hair, copper green scales for skin, golden wings on her back, and eyes that would turn anyone that met them to stone.
The story in which the hero Perseus slew Medusa is famous even in Japan. It's said that Perseus' divine blade Harpe sheared through Medusa's scaley skin, and Medusa's head contained the power of petrification even after being severed.
Medusa was originally a goddess, but a curse by the goddess Athena lowered her to a monster. The story goes that Medusa, overly proud of her own beauty, desecrated a temple to Athena.
According to the Theogony describing the genealogy of the Greek gods, Medusa had a pair of twin older sisters named Stheno and Euryale. Together they were known as the three Gorgon (derived from ancient Greek gorgós, meaning "dreadful") sisters.
It is written that the aboriginal Pelasgians worshiped Medusa when she was still a goddess. After the Pelasgians were conquered by other groups, Medusa was demonized from a great earth goddess into a monster.
After becoming a Heroic Spirit, she retains her wholesome beauty even while possessing the Mystic Eyes of Petrification. Her flying steed Pegasus was said to have been born from a piece of her severed head, or created by her blood when it dripped into the ocean.
Medusa specializes in close combat, taking advantage of her high Agility and Monstrous Strength. Her weapons of choice are chained daggers. While quite lethal, they are not Noble Phantasms, so it is difficult to deal critical blows with them when fighting Servants. Medusa excels in a fight using Noble Phantasms. She can annihilate many enemies at once using her anti-army Noble Phantasm Bellerophon (Bridle of Chivalry), which requires an enormous amount of prana. Additionally, she has several methods of attack that can be adapted to a wide variety of situations, such as Blood Fort Andromeda (Outsider Seal - Blood Temple) which can close off a targeted area and dissolve the humans inside to obtain prana, and Cybele (Mystic Eyes of Petrification) which is normally sealed beneath her blindfold.
Magic Resistance: B
Originally a Greek goddess that turned into a monster, she possesses extremely high Magic Resistance. The Magic Resistance ability that the Rider class possesses is weaker than that of the three knight classes, but the fact that it remains so high is thanks to Medusa's inherently high Magic Resistance.
Rank B Magic Resistance easily nullifies spells of three bars or less. It is also difficult to damage Medusa with even high thaumaturgy and grand rituals. Moreover, Medusa herself is skilled in sorcery, so she has countless ways to counter enemy spells. In other words, Medusa is excellent at fighting magi. Backing all that up is her Agility. Compared to Lancer who boasts the same level of "speed" she is inferior in sudden bursts, but has more endurance. Even with powerful spells, it is extremely hard to hit Medusa who will keep moving at high speed.
As might be expected of the Servant Rider, Medusa posses a high rank in the Riding ability. But instead of covering only specific animals and vehicles, her ability extends to nearly all beasts, including Phantasmal Beasts and Divine Beasts. However, even with an excellent Riding rank, she cannot ride dragons.
The steed that makes best use of Medusa's Riding ability is the legendary winged horse Pegasus. Mastering a winged horse of the Phantasmal Species is a difficult accomplishment even for Rider-class Servants. When riding the heavenly mount that was born of her own blood, both rider and steed move as one.
When the Noble Phantasm Bellerophon is used, all of Pegasus's abilities are drastically increased for a limited time. Even in the story, this combination was able to toy with Saber by attacking from the skies, forcing her into a difficult battle.
Be that as it may, the only time Medusa was able to use Bellerophon was in her fight against Artoria. During the modern Holy Grail War which unfolded in a sprawling city, Medusa had few opportunities to use her Riding ability. Perhaps because of this, she was eliminated without sufficient chance to use her power.
Mystic Eyes: A+
Eyes that originally function to collect visual information are instead a method to impart magical effects upon a subject. Medusa possesses the Mystic Eyes of Petrification "Cybele", which are Mystic Eyes of the highest order. As in her legend, subjects caught in her vision will be petrified if their Magic Resistance is C or lower.
Caption: Medusa's Mystic Eyes of Petrification cannot be replicated by magic.
Independent Action: C
The ability to materialize independently. If the Master is lost (or if the contract is nullified), Medusa can continue to materialize for one day. Since Medusa can also obtain prana by drinking blood, she can prey on humans as the situation demands and extend this duration.
Caption: Thanks to Independent Action, Medusa was able to return to Sakura after Matou Shinji was defeated.
Monstrous Strength: B
Medusa has this skill due to her property as a monster despite being a Heroic Spirit. If used, she can temporarily magnify her Strength. This is a necessary skill to Medusa, who does not fight with Noble Phantasms in close combat. It has many uses with her chained daggers, such as forcibly dragging an impaled opponent around.
Caption: Medusa possesses high close combat ability thanks to Monstrous Strength.
In Greek mythology, Medusa was once a beautiful goddess, so she originally has very high Divinity. However, because she later became a monster, she is an unusual being possessing the "monstrous" attribute that is antithetical to normal Heroic Spirits. Thanks to this, Medusa manifests as both a goddess with Riding, and a monster possessing Mystic Eyes, Monstrous Strength and so forth. Nonetheless, her aptitude as a goddess Divine Spirit is mostly degraded.
However, her Divinity has not been completely eliminated, and it appears that she still has it.
Maximum Targets: 1 Person
One of Medusa's anti-personnel Noble Phantasms, Breaker Gorgon is the counterpart Bounded Field thaumaturgy to Blood Fort Andromeda. While the target's consciousness is sealed within Medusa's mind, the target can be prevented from activating any abilities. Also, Medusa primarily uses somewhat weak Noble Phantasm not on others, but on herself to seal her Mystic Eyes of Petrification that are normally always active.
Though mainly used to seal Mystic Eyes, its original use is obviously to exert a degree of mind control on the opponent. Actually, when she took control of enemy Master Emiya Shirou's mind and gave him erotic dreams, she was actually trying to extract prana from him. Also, it requires only a steady stream of prana to have an effect. It is difficult for individuals with low magic resistance to notice it, much less avoid it.
Maximum Targets: 300 People
Medusa's most powerful attack. It is not a Noble Phantasm to be used in a one-on-one fight, but rather an "anti-army Noble Phantasm" that can cut down enemies in a wide area. An uncommon type of Noble Phantasm consisting of a paired bridle and whip, it can rapidly augment the abilities of Phantasmal Species. In battle, the strengthened Phantasmal Beast and Medusa fight as one.
Bellerophon is extraordinarily powerful whether used defensively or offensively. However, during the Holy Grail War, it could not display its merit when matched against the more powerful Noble Phantasm wielded by Artoria. Nonetheless, the fact alone that it temporarily withstood a full frontal attack by Excalibur is evidence of its great power.
Blood Fort Andromeda
Maximum Targets: 500 People
One of Medusa's Noble Phantasms, Blood Fort Andromeda is the counterpart Bounded Field thaumaturgy to Breaker Gorgon. It takes the form of magic circles placed as predetermined points. Immediately following the placing of these circles, people within the affected area will only have their life force sapped slightly, but once the field is activated, they will be quickly dissolved. However, it is difficult to affect people with knowledge of thaumaturgy. Thus, it is not a Noble Phantasm to be used for combat, but rather a Bounded Field created by Medusa to efficiently gather blood to drink for prana.
In the Holy Grail War, Blood Fort Andromeda was deployed at Homurahara Academy and activated. From outside the field, the school appeared normal as usual, but the interior was stained with blood, as its name suggests.
- Fate/Grand Order material IV - Gorgon, p.352-363, translated by castor212 at Beast's Lair.
 Fate/side material - Encyclopedia: Self Seal - Temple of Darkness [Noble Phantasm], p.062 [T]
Self Seal - Temple of Darkness [Noble Phantasm]
A Mystic Eye Killer applied to oneself.
It was the blindfold of Rider's, which sealed the excessively powerful Mystic Eyes into a boundary field (another World).
Shirou's erotic dream in Sakura route was a result of being drawn into this Temple of Darkness. In this World, Shirou had an erotic encounter with Rider in disguise, and had some of his blood and semen drained.
Anyways, the blindfold had meaningful purposes.
"Hmmm, was the blindfold just for style and fun points?"
 Fate/complete material III: World material - FAQ with Nasu: Noble Phantasms, p.134
Q: Rider's eye cover, Breaker Gorgon, does it allow her to see what's before her?
Q: Rider's eye cover, Breaker Gorgon, does it allow her to see what's before her? Also, the Mystic Eyes Killer Rider is wearing, does it have a comparable sealing power as this Noble Phantasm?
A: Her sight is completely sealed. Although Rider perceives the world through senses such as hearing, touch, smell, and magic-detection, she is able to acquire more precise information than sight.
About the glasses, it only seals the power of petrification. It does not act like the Breaker Gorgon, which "seals all magical properties".