Index Librorum Prohibitorum's John's Pen Controller allows one to freely use any of the 103,000 grimoires in Index Librorum Prohibitorum. The user can use all of Index's spells even if she is not present. Each grimoire gives the user access to a specific spell and book in the library that they can mentally search through and activate. Each time a spell is activated it is accompanied with Index's voice warning and grimoire name. Grimoires are poisonous to the mind, however Fiamma seems to be immune to the mental destruction, decay, insanity, and other debilitating effects that result from reading a grimoire original.

“I suppose you could call it the external controller for John’s Pen, the safety device installed on Index Librorum Prohibitorum. It is a treasured item held only by the heads of the royal family and the Anglican Church. Of course, there’s the contamination from the original grimoires, so it’s really only a last resort. ...Didn’t you find it odd? No matter how much she wanted it herself, did you really think they would leave the Index Librorum Prohibitorum that held the 103,000 grimoires in the middle of that city of science with no insurance whatsoever? And it was the Archbishop that created this cruel system.” The second was what Index said herself. “Yes, I am the grimoire library...belonging to Necessarius, the 0th Parish...of the Anglican Church. My formal...name is Index Librorum Prohibitorum, but...as a nickname it is abbreviated to...kssssshhhhhhhh.” Index had been muttering expressionlessly, but she had suddenly started to shake unnaturally before she collapsed to the ground. “Index!!” “Oh, was John’s Pen damaged at some point? Well, it’s unfortunate that the body can’t be completely controlled, but it’ll still work out like this. ...If I modify the spiritual item a bit to increase its ‘output’, I can most likely manage to freely access the knowledge of the 103,000 grimoires.”
(Toaru Majutsu no Index Volume 18, pg 239)