The Wicked Book

"When a person thought of evil plans, they took extra care to examine such ideas, then took them to the others, where flaws and problems were pointed out and, like a stone, it was polished over and over again until not a single part of it could go wrong." 
"What happened then?"
"They were spread throughout then land and placed in a way that many people found attractive. More money. More fame. More everything."
"Why isn't the world much worse off then?"
"Because a group of wise people wandered, and every time they found something like that, they took it and placed it in a relic they carried with them, a book that would absorb ideas and create chapters with them." 

— the origin of the Wicked Book, a relic thankfully lost to time

Mechanics & Inner Workings

The story of "The Wicked Book" differs slightly depending on what culture is telling it. Sometimes the pages are made of paper, other times leather, and in one story they are clay tablets bound together with twine. 
Main components of "The Wicked Book" seem to be: pages that can be added on, binding which can take in as many new pages as needed, and strong spells to make sure it could never be found again*. 
*In some cultures, the book has been found, the results have been devastating, and the book was lost again**. For example, the wave that destroyed the sea-coast town of Meridas was an example of a spurned sea elemental gaining control of the book for a while. 
**This is why it's thought to be a myth and not an actual book.

History

This book was made with a mixture of different spells affecting it at each stage of its construction. There were spells to bind the relics of thought (usually found in a solid, concrete form that could be picked up), spells for the book to be ignored and not acknowledged by people, and even more spells that, when cast, kept a person from opening the book after it had been closed.   It took a magic user with a high endurance to do this, and someone to whom magic is almost their native tongue. Such people are rare nowadays, or in the case of families who have kept the magic bright within them, are not willing to undergo tests to see how well they do (or do not) wield their magic.

Significance

Scholars are unsure if this is a world-creation myth or a real life event. However, the significance of this book (if it could ever be found) would mean that people were able to wield magic to a greater extent than currently possible/known.

Creation Date

Unknown

Rarity

A duplication of this book is impossible due to the high amount of near constant magic needed, and if a person were to repeat the actions of the first party, then the contents of the book would change.