When this book is opened, its text is very specific to the reader, and whoever opens this book is compelled to read it, and read it aloud. Only those of strong will can stop themselves.
The book is inhabited by a rather powerful spirit, which will not only manifest the reality which is read, but will also put the book right back where the adventurers drop it.
Closing or destroying the book will stop the manifestations, but the book will be restored by the spirit a short time later. Beating the manifestations will drain the spirit enough that destroying the book will leave it permanently unable to restore it. Killing the reader will prevent them from continuing.
The reader will be startled to find the book starts off describing the situation that they currently find themselves in. For instance “When planning an eventful evening, it is important to first put the group at ease, take for instance the would-be heroes who stood in the dark mage’s library. One of them, the most curious, stood, holding a book not meant for his eyes, and read the text aloud to his friends. He spoke of things creeping in the darkness, to descend upon them…” and at this moment the ‘things’ would descend upon them.
The spirit is diabolical, but it does not have unlimited power. It may manifest only a few creatures, and will therefore attempt to match the creatures to the weaknesses of the individual adventurers. The creatures summoned will attack their target, but may be engaged in combat by anyone else, and will defend themselves. This is the best way to combat the spirit, which cannot be harmed directly.
The book was originally made by a dark wizard, who placed it as a trap in his library, which lay on the way to his vault, which he found was being frequented all too often by thieves (ie, heroes). Even if he did deal with the thieves (heroes) himself, he didn’t like to have to keep doing it. So, he placed this in the most valuable looking place in his library, placed inside a glass case and all, to make it look as important and valuable as possible.
When confronted by this book, a thief (hero) could barely stop themselves from taking it, and attempting to take a quick gander at its contents. This was usually the last of the thief (hero) as far as anyone else was concerned.
While this is mostly good for an encounter, I can think it might be a good way to get the players thinking about matching their strengths to their enemies weaknesses, if an enemy does it to them.
Or, perhaps the item is in a dungeon just unearthed. Perhaps the local ruler has sent an expedition down there and nobody has returned. Not wanting to risk any more of his subjects’ lives, he turns to the adventurers to help him out. It might turn out that the expedition came across this book, and couldn’t help themselves. The adventurers would find the bodies of the expedition, and the book sitting there, in the same room, looking untouched. Perhaps it would be an idea to have the beginning of the book written out for the unlucky player who ends up reading the book, and have them narrate the battle until they either win, or lose, or the hero manages to gather the willpower to close the book.
The book will compell the reader to continue reading, and read aloud, the story of his group’s demise. He, or she, may do nothing other than read the book unless they find the strength to wrench themselves away. The story will attempt to become fact while the reader still reads. The creatures created are real in every way, but will vanish once the story is done. The reader is quite aware of what is going on around him, but cannot take his eyes off the story.