Prelati's Spellbook

Prelati's Spellbook: Text of the Sunken Spiraled City is a demonic guidebook and summoning grimoire with a cover made of glistening and wet human skin. It allows the summoning of an army of Horrors (海魔), water demons from the depth of the seas of another dimension, High Thaumaturgy, and Greater Rituals. Rather than simply functioning as tome of spells and curses, the book itself is a "monster" that functions as a magical energy core possessing its own enormous magical energy furnaces completely independent of its wielder's magical energy. It can either be read from as a regular tome for minor spells, such as summoning a single demon with "", 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.

Depending on the verses used, this allows for the usage of a class of ritual magecraft that tens of magi couldn't accomplish together. 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. It is also known as the in reference to 's. It was probably copied from the original book in the sunken spiraled city, translated from the R'lyeh language into Chinese during the, and later translated into Italian by François Prélati.

The water demons, without a discernible torso nor appendages, are best described as masses of tentacles with circular mouth-like openings marked with shark-like blades. Their innards spew a mist-like gas that would cause lung corrosion in a normal human. They are neither spiritual bodies or members of the Phantasmal Species, but rather creatures from an entirely different dimension with different natural laws. 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. They can be guided, but the tome doesn't allow for direct control over the creatures. It can be said that it is "inviting" them, simply utilizing the magical energy and technique to open the door into the world.

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, as the flesh of the defeated demons will instantly spawn new ones to take their place. 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. If the connection is severed for even a moment, they will all instantly dissipate into the blood of the original sacrifices. If Caster is in need of an escape route, he can surge magical energy from the Spellbook into the blood that is still receiving its energy due to the summoning ritual in order to cause it to boil and evaporate into a bloody mist.

Gigantic Horror
Upon fully forming, the 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, of which all of the summoned familiars had been bits and pieces, that is at least the size of a building when only considering the mass of it not obscured by the river. 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, 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.

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.