Prelati's Spellbook

Prelati's Spellbook is demonic guidebook with a cover made of human skin. It allows the summoning of water demons from the depth of the seas.

It isn't just a bunch of papers with curses written on it which Caster can read: the book itself functions as a prana core which it uses freely. It isn't connected to the user's prana. It enables the use of high-level magecraft and ritual magecraft.

It doesn't actually allow direct control over the water demons it summons, but it allows the usage of a ritual magecraft class that tens of people couldn't do together, depending on the verses pronounced.

It is based on the R'lyeh text from the myth of Cthulhu. It probably was copied from the original book in R'lyeh language, translated into Chinese in the Xia dynasty from the sunken spiraled city, then translated into Italian by François Prélati. Of course it would be able to summon fantastic aquatic creatures.

François Prélati was known for being an adept of black magic, along with his friend Gilles de Rais.

Source: the dictionary resource at Jonnobi and the status page in Fate/Zero.