“Could it be…”

Although it was a most pessimistic guess, but it probably shouldn’t be wrong.

It was summoning magecraft that summoned these countless demonic monsters, resurrected them, and urged them to relentlessly rush towards Saber’s sword. And the one reciting the spell was none other except that book of magecraft.

It isn’t merely a pile of paper which recording spells. That book is probably a prana furnace with an incredible capacity, a ‘monster’ that can use magecraft just by its own power. Caster wasn’t reading the spells from the pages of the book, but just freely manipulating that book which served as a prana source.

Prelati’s Spellbook • The Text of the Spiralled Sunken Citadel – it is indeed a terrifying Noble Phantasm. Had Irisviel been Saber’s proper Master, she should have seen through his abilities the first time she met Caster and recognized that the opponent is an extremely dangerous Servant with an ability to modify the power of his Noble Phantasm. Had she knew of it, Saber would definitely not have fallen so easily into his trap and locked in a dragging battle with Caster even if she’s scorned for being a coward; she might have acted more prudently.

- Fate/Zero: Act 6, Part 3