Blood Fort Andromeda: Outer-Seal - Blood Temple (他者封印・鮮血神殿, Tasha Fūin - Senketsu Shinden?) is the Temple of Blood that surrounded the Shapeless Isle which was the home of Medusa and her sisters, turning it into a palace of darkness, dyed crimson from blood, where the sun didn't shine.
The temple, known to petrify those who embark on the island and devour them, was a boundary field constructed by the Mystic Eyes Medusa possessed. By magnification of the Mystic Eyes' projection, a fixed conceptual field is substituted into "the inside of her eyes", and the life force of those inside it is plundered. An absorption power that literally "dissolves the body" of ordinary persons without any Magic Resistance. Even Heroic Spirits will have their life force plundered as long as they are inside the barrier. Medusa is able to implement it as a Bounded Field of blood that she uses to primarily absorb magical energy by dissolving any caught live humans and reducing them into red blood and organic ooze. Using her blood as a material component, she sets up a number of sources around an area, taking the form of magic circles placed as predetermined points, to slowly build up energy without leaving a single trace of her existence. It is impossible for Rin to completely destroy it as new circles constantly appear and old ones are restored. She can only delay its operation date by constantly removing as many circles as possible. It takes over ten days to completely store enough energy to activate the full power of the field, but it can still be activated at any time once the foundations are complete. It damages the leylines of the area once it is completely set up, so it cannot be reactivated on the same land twice in succession. It leaves a general listless feeling around the area, and Shirou senses the barrier by seeing an image of the area covered with membranes and the people looking like empty dolls while smelling the sweet scent of honey from an insectivorous plant.
Immediately following the placement of these circles, people within the affected area will only have their life force sapped slightly. Once activated, it forms an advanced Bounded Field almost on the level of Magic. It creates a crimson dome-shaped Bounded Field resembling a gigantic red eyeball around the area of operation, and it tints the area in crimson red that makes a person think that blood is seeping into their eyes. The barrier acts like a separate plane that completely cuts the affected area from the rest of the World, cuts off all escape routes for fleeing enemies, and Rin is left unable to contact Archer. As the barrier is conceptual, if it's pushed too far it will actually transform into the Shapeless Isle itself, which would be ironic for Shinji to invoke the place Perseus escaped.Naturally, from outside the Bounded Field, the area appears normal as usual and the red sky and dome cannot be seen at all, but the interior is stained with blood, as its name suggests. Victims within the barrier are quickly dissolved into blood as if they were inside a stomach upon activation, and its magical energy is given to either Medusa or her Master. Those with a proper amount of magical energy or with knowledge of thaumaturgy are able to resist the Bounded Field, such as a Servant or a magus respectively, but those without any are instantly drained. Even then magus can only resist Blood Fort Andromeda for a short amount of time. 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 mana. If the Bounded Field is activated without the proper energy, it can take up to a number of minutes to completely dissolve the victims. If Medusa has a stronger master, such as when switching from Shinji to Sakura, the effect of the field will also become stronger. It is completely controlled by Medusa, and it will continue to stay active even after she dies. It will only be deactivated through her own will or once her body dissipates.
Those under the effect of the weakened barrier almost instantly collapse unconscious and begin to convulse and foam at the mouth due to the dense air. Stronger people can resist it for a small amount of time, while weaker people will instantly have their skin start to melt, have inflammation resembling a keloid, and their eyes will look like those of a dead fish. Those closest to the origin of the barrier will suffer the most severe effects, such as their blood being instantly vaporized and having the look of pale wax dolls piled like corpses. Someone with a slight resistance, such as Shirou, will feel nausea, dizziness, a sensation of being extremely hot, and instant discomfort as if their senses were revered. They will have trouble breathing as if the oxygen had suddenly dissipated. If the barrier is removed quickly enough, those furthest away from the origin will wake up as if they were sleeping and get away with a stay in the hospital for what amounts to severe malnutrition and slight memory loss. Those closer to the source however will suffer from side effects from having their life force drained, including scars and necrosis on the tips of their limbs from the melted skin, physical disabilities such as being on the verge of blindness, and mental scars from oxygen deprivation.
It was shown in the Heaven's Feel route that Blood Fort Andromeda can affect servants as shown when it began to drain mana from Archer this was due to Rider being fed a sufficient supply of mana by her Master Sakura and was able to return to being at full power. Due to this it's believed that, unlike the weakened Blood Fort Andromeda in both the Fate route and Unlimited Blade Works, it's the version in Heaven's Feel that is the true strength of the Blood Fort Andromeda. As Servants and humans are so easily effected by the stronger Blood Fort Andromeda, it's believed that only Servants such as Caster or Berserker who both have large stores of mana are able to resist the Blood Fort Andromeda for a long time, but even they would succumb to the Blood Fort at some point.
Rider explicitly stated in Fate/hollow ataraxia that she's capable of summoning Blood Fort Andromeda on the scale of Fuyuki City.
In Fate/Grand Order, Blood Fort Andromeda is utilized as a Personal Skill, with decreased output values due to not being a True Name release.
Pandemonium Cetus is a strengthened version of Blood Fort Andromeda, which does not require a set up time and can be activated with just the release of its True Name. Its effectiveness is lowered somewhat against inorganic-type opponents.
 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 book I - translated by castor212 on Beast's Lair
 Fate/side material - Encyclopedia: Outer-Seal - Blood Temple [Noble Phantasm, Magecraft], p.066 [T]
Outer-Seal - Blood Temple [Noble Phantasm, Magecraft]
Blood Fort Andromeda.
A boundary field of blood. Any humans captured within it will be dissolved.
The dissolved humans are reduced to red blood, which is consumed by the boundary field's user. Its primary purpose is the absorption of magical energy.
It was used once in every route, and by the end of the game was like an old familiar friend.
Since the boundary field is in the shape of a dome, the school building looked as if it was captured within an enormous eyeball.
Naturally, from outside the boundary field the red sky and dome cannot be seen at all. It's just like Rin said: any boundary field that reveals its strangeness to the outside world is third-rate at best.
Also, because the Bloodfort seriously damages the leylines where it is used, it cannot be used on the same land twice in succession.
- Unlimited Blade Works Animation Material I - Kinoko and Takashi Q & A.