Vatican Library

The Vatican Library (バチカン図書館), called the Biblioteca Apostolica Vaticana in Italian, is the library of the Holy See, currently located in the Vatican. It is one of the oldest libraries in the world and contains one of the most significant collections of historical texts. The Roman Catholic Church somehow acquired the Book of the Law and locked it inside the library.

Index Arc
Stiyl Magnus mentions that Index has been to the library prior to the start of the first volume in the timeline in addition to Mont Saint-Michel Abbey, the Pataliputra ruins, Château de Compiègne, and Louvre Museum. There are one or more grimoires sealed in the library which cannot be taken away.

World War III Arc
During World War III, Matthai Reese gains access to the great library beneath the Vatican in order to analyze the spells that Fiamma of the Right had used for his plan. He is later contacted by Vasilisa to inform him that she was able to free Russia's young Patriarch for access to the Russian Orthodox spells and to coordinate their attacks on the Star of Bethlehem, Matthai had reached a point where reading through the spellbooks pained his mind, but when a young priest come over to help him, Matthai pushes the priest away, and reaffirms to him that they must go through with this in order to open a path for a hope-filled tomorrow.