Prelati's Spellbook: Textbook of the Sunken Spiral Castle (螺湮城教本
, Rain-jō Kyōhon
Purerātīzu Superubukku
?) is a demonic guidebook and summoning grimoire with a cover made of glistening and wet human skin.[1][2] It is also known as the R'lyeh Text in reference to H. P. Lovecraft's Cthulhu Mythos. It was probably copied from the original book in the Sunken Spiral Castle, translated from the R'lyeh language into Chinese during the Xia Dynasty, and later translated into Italian by François Prelati; though an alternate telling of the grimoire's origin describes Prelati sealing a link to the Sunken Spiral Castle into a blank Bible, turning it into a Mystic Code capable of summoning Horrors.[3] Furthermore, the previous original text was described in a language other than that of humanity's languages. The book contains records of a forbidden knowledge relating to an evil god of an ancient time sometime before humanity's origin.[1]


François PrelatiEdit

The result of Prelati utilizing magecraft while dispelling their own rationality with drugs they compounded themselves. Against astronomical odds, it ended up linked to a place one must not link to, Prelati inscribing the truth of it in a blank bible that was a Mystic Code and sealing the link itself. Prelati was therefore incapable of reproducing it, and entrusted the Spellbook that is capable of opening the one and only link to a knight who was their sworn friend. Prelati can never use the Noble Phantasm unless the knight returns it on the spiritual level, but will the day of their reunion ever come?[3]

Gilles de Rais (Caster)Edit

English | Japanese

« Let me show you the greatest Coooooool!"
"Fufufuhahahahaha!! Ahahahahahahahahahaha!! »

(Caster, Fate/Grand Order)

Although not a magus in life, Gilles received the spellbook through association with Prelati. It allows the summoning of an army of Horrors (海魔, Kaima?), water demons from the depths of the seas of another dimension, as well as High Thaumaturgy, Greater Rituals and ritual magecraft, regardless of the user's skill in magecraft in relation to the use of the grimoire.[2][4] Rather than simply functioning as a tome of spells and curses, the book itself is a "monster" that functions as a Magical Energy Reactor (魔力炉心, Maryoku Roshin?) core possessing its own enormous magical energy furnaces completely independent of its wielder's magical energy.[5] It can either be read from as a regular tome for minor spells, such as summoning a single demon with "Cthulhu fhtagn", or it can be completely commanded by the user, causing the pages to rapidly flip by themselves as the tome provides the necessary incantations and its own independent power to conduct the summoning spells with an inexhaustible magic energy supply.[5][6]

Depending on the verses used, this allows for the usage of a class of ritual magecraft that tens of magi couldn't accomplish together. Even if the user is not a legitimate magus and has no groundings in magecraft like Gilles de Rais himself, this grimoire, acting as the agent, can exercise summoning magecraft to compensate, allowing one to become a specialized summoner with this Noble Phantasm for as long as one possesses it.[4][5] For this reason, Gilles de Rais can summon the sea demons and employ them.[4] The Spellbook itself is constantly giving off magical energy, and if it is cut, rather than showing pages, it shows a glowing, unearthly substance. It is capable of regenerating damage caused to it under its own power, and the "wound" closes much like it would on a living creature.

  • Prelati's Spellbook after being cut by Gáe Dearg
  • Horrors spawning from other defeated Horrors

The water demons, without a discernible torso nor appendages, are best described as masses of tentacles with circular mouth-like openings marked with shark teeth-like blades. They usually are shorter than a man but can also be summoned several times larger than one. Their appendages are heavily spiked, serrated and covered with either eyes, suction cups, spines, papulae or all of them. Each tentacle contains an additional mouth which split open to reveal them, and have some form of elasticity to their limbs to allow them to reach and bind their target from a distance. They also possess some regenerative powers to non-lethal wounds, even crippling ones such as lost limbs. Their innards spew a black mist-like gas that would cause lung corrosion in a normal human, and can spit a purple, viscous acid that is also a paralyzing agent.[7][8] They are not living beings of nature, but creatures from an entirely different world with different natural laws, containing attributes in accordance with the deep sea system. They are summoned by using the blood and flesh of creatures, living or deceased, as a sacrifice, and they must constantly receive magical energy from the Spellbook. While one sea demon's fighting strength is generally not comparable to a Servant's, the Noble Phantasm's Anti-Army classification is not for show as it makes it possible to summon dozen of bodies in order to further compensate for an individual demon's fighting weakness with overwhelming numbers.[4]

It is able to endlessly call them, revive them, and send them at any enemies as its wielder commands it. They will stay summoned as long as they receive magical energy, allowing Caster to leave them as guards to his workshop while he is away. No matter how many are destroyed in close combat, their numbers will never decrease for it is possible that the flesh of the defeated demons, acting as mediums, will instantly spawn new ones to take their place; because of this, the Noble Phantasm has an excellent ratio of magical energy consumption to the amount of summoned demons.[4] Enough are created that both Saber and Lancer working together are able to kill five hundred each without there being a single chance to break through and reach Caster just outside of their battleground. However, it is the "grimoire that is persistently performing the summoning ritual", and the summoning itself is not a "miracle of the Noble Phantasm." The demons summoned through the grimoire cannot remain present in the current world without a continuous connection to the book's magical energy supply. If the connection to the supply is severed for even a moment, they will all instantly dissipate into the blood of the original sacrifices.[4] If Caster is in need of an escape route, he can surge magical energy from the Spellbook into the blood that is still receiving the book's energy due to the summoning ritual in order to cause it to boil and evaporate into a bloody mist.

Gigantic HorrorEdit

  • Caster starting the ritual
  • The fog forming around him
  • The tentacles swallowing Caster
  • The rest of the demon swallowing Caster

Caster also has the potential ability to become absorbed with the Spellbook into a mass of demons to employ a Gigantic Horror (巨大海魔, Kyodai Kaima?). He needs a good deal of preparation time, and beginning with the chant "Ph'nglui mglw'nafh wgah'nagl fhtagn", he summons numerous horrors into the Mion River. They begin to gather under his feet and form a "sandbar", which is made up of an extremely numerous amount of them due to being in the middle of a deep river. There is a strange fog that forms around the area as an aftereffect while he simply stands relaxingly in the middle of the river. The vortex of raging magical energy overflowing from the tome begins to distort even the space around it. The Horrors, ever increasing, begin to swallow him with their tentacles, covering his entire body. Swelling in numbers, they begin to entwine and coalesce while rising out of the water as a large lump of meat. It can eventually even be called an "island of meat", but still grows even then.

  • The Gigantic Horror's Anime Design
  • The Gigantic Horror's Novel Design

Upon fully forming, the really enormous Gigantic Horror becomes an aquatic giant befitting the name of a "sea demon", the nightmare that governs the ocean in the realm outside of the world. It can be called the true form of the diabolic underworld, a massive extradimensional evil god that is at least the size of a building when only considering the mass of it not obscured by the river, of which all of the summoned familiars are bits and pieces of it. It has a disgusting, yet overwhelming majesty, and while Caster is no longer visible, his voice is still able to ring out from it. While magecraft can be called "the art of flipping through evil", the Gigantic Horror can be called "genuine evil", the incarnation of something which has the craving desire of devouring with thorough greediness. The act of summoning such a being cannot be called an "art" or anything else. It is not under his control in any way, only invited to eat the city that he neglects, which would only take it a few hours to consume. It was brought forth with the intention of suicide. As it feeds on lives to grow, it will eventually grow too large and die, so it was summoned only for the purpose of destruction.[9]

With a similar structure to an amoeba, it has no bones or organs and it feels no pain, even with a third of its body blown away. It continuously regenerates, no matter how much damage is done, allowing it to survive an assault from Saber, Lancer, Rider's chariot and the force of his entire army in Ionioi Hetairoi, and mountain-shattering weapons fired from Gilgamesh's Gate of Babylon all at once without suffering any permanent damage. It has tentacles capable of swatting an F15 Fighter Jet out of the sky with ease, and after it crashes into the demon's body, the F15 is swallowed by it without leaving a trace. In order for it to be destroyed, either Caster's Master must be killed, allowing Caster to potentially fade away naturally after his magical energy supply is used up unless the creature starts feeding on humans, or an Anti-Fortress attack, such as Excalibur, must be used in order to eradicate the entirety of its flesh and kill Caster deep inside the heart of the creature. Though from another dimension, it is affected by weapons with an affinity towards slaying monsters.[10]

Gilles de Rais (Saber)Edit

Although Gilles possesses this Noble Phantasm in the Saber class, he cannot use it. If he were to use this Noble Phantasm, he would automatically class-change to Caster, and Saint War Order will be lost.[11]


  1. 1.0 1.1
  2. 2.0 2.1 2.2 2.3 2.4 2.5
  3. 3.0 3.1 3.2 3.3 3.4 3.5

  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Fate/Grand Order material I - Gilles de Rais (Caster), p.288-293
  5. 5.0 5.1 5.2 5.3 5.4
  6. Fate/Zero - Episode 02
  7. Fate/Zero - Episode 7
  8. Fate/Grand Order - Sea Demon/Fiend enemy.
  9. Fate/Zero - Einzbern Consultation Room 4
  10. 11.0 11.1 11.2 11.3 11.4 Fate/Grand Order material I - Gilles de Rais (Saber), p.082-091, translated by RanonCap at Beast's Lair.

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