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.
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.
[v] Fate/side material - Encyclopedia: Self Seal - Temple of Darkness [Noble Phantasm], p.062 [T]Translations by food
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?"
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".