Testament of Carnamagos (テスタメント・オブ・カーナマゴズ) is an ancient, magical text that is considered so dangerous, it has a Type 1 warning attached to it. Even skimming the text can affect the reader in a negative manner. The original copy resides in restricted are of the library at the College and looked after by the librarians who conduct routine inspections of the important text to be sure they are not being misused. A copy of the book is kept in the English branch of the College, which is considered a weaker version of the original.
Marielle told Elias about the book in Episode 21 in order to transfer the Dragon Curse in Chise's arm into Stella. Torrey granted Elias access to the restricted area of the library at the College, where he read and memorized the necessary passage needed to perform the spell.
The book appears as a flash during a bushcraft exercise trip in Chapter 60 and implies that it was used to drain the magic energy from Lucy, which nearly kills her. The book is again used in Chapter 63 to do the same thing to Simeon Paladilhe, causing him to become bed ridden for a long period of time.
In Chapter 63, it is discovered by the librarians at the College that the book taking up the spot where the Testament of Carnamagos is supposed to be, is a fake after it disintegrated to dust, leaving only a spider web behind as a clue.