Explanation of John's Pen Controller

John's Pen Controller allows one to freely use any of the 103,000 grimoires in Index Librorum Prohibitorum. Mastery over the list of prohibited books is said to allow the user to twist the laws of the world to their will without exception, effectively gaining power akin to a Magic God. The holder of John's Pen Controller 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, body and soul and can cause instability likened to a computer system forcibly running a foreign program. The Holy Right seemingly allows Fiamma to ignore the debilitating effects that result from reading a grimoire original.

“My brain and spirit are protected by religious barriers, and magicians who attempt to exceed being human must exceed the boundaries of their own common knowledge to arrive at the desired state of mind which could almost be likened to a type of insanity. However, for a normal person from a weakly religious country like Japan, it could all be over after just casting one more spell.”
(Toaru Majutsu no Index Volume 1, pg 97)

She had 103,000 grimoires in her head. Just reading one of them could drive you insane and yet she had put each and every letter of all those books in her head. How much pain had that process caused her?
(Toaru Majutsu no Index Volume 1, pg 98)

She was a library with perfect copies of the world’s 103,000 original grimoires in her head. To obtain her was to obtain all the magic in the world. “...Right.” From her voice, it sounded like she was about to die. “With the 103,000 grimoires, you would be able to twist everything in the world to your will without exception. That is what we call a Magic God.” Not the god of the demon world, but someone who had thoroughly mastered magic to the point of entering the domain of a god. A Magic God.
(Toaru Majutsu no Index Volume 1, pg 101-102)

It had only been a few characters. He hadn’t looked directly at them; they had only barely entered his field of vision. And yet his brain felt like it was going to split in two. This wasn’t like a watered-down copy altered and interpreted for general use. It was a true original grimoire.
(Toaru Majutsu no Index Volume 15, pg 126)

The first was what Fiamma said next. “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 237)

A grimoire was a book filled with magical knowledge. The ones known as original grimoires were especially dangerous. They held such high quality foreign knowledge that they were said to “contaminate” the brain of anyone who read them. In all seriousness, they were able to forcibly demolish a human’s personality.
(Toaru Majutsu no Index SS Norse, pg 184)

“Hmm. Come to think of it, that’s pretty risky too,” said Birdway lightly. “Despite the fact that reading an original grimoire is said to destroy your personality, there are people who go out of their way to create copies. Do you know what happens when they do? Towards the end, they seem to faint while they foam at the mouth, but their fingertips continue to move like some kind of machine.” “Yes, but it seems certain people with a special disposition have a natural mechanism in their brain that defends against the contamination from an original grimoire.” “Like the Index Librorum Prohibitorum? That too is pretty risky. Just how many defensive mechanisms does she have put inside her? You can call them religious defensive walls all you want, but one wrong step could modify her mind to the point that she loses all her functionality as a human dozens of times over.”
(Toaru Majutsu no Index SS Norse, pg 184)

(This is just like from an original grimoire...) Grimoires with knowledge of too great a purity known as original grimoires could shatter a human’s mind just by being seen. It may have been similar to creating instability over an entire computer system by forcibly running a program meant for a different OS.
(Toaru Majutsu no Index SS Norse, pg 243)

The greatest of those were helping to found the world’s greatest cabal and translating original grimoires such as The Kabbalah Unveiled, The Book of the Sacred Magic of Abramelin the Mage, and The Greater Key of Solomon. The originals were not just incomprehensible but also shattered the reader’s soul with their toxic knowledge, so he converted them into a simpler form that anyone could read.
(Shinyaku Toaru Majutsu no Index Volume 21, pg 69)