Genre: High fantasy, possibly including travel between parallel dimensions, possibly with Dark Age Call of Cthulhu undertones.
Scene: The grimoire has sat in the largely forgotten dim high library in the east tower of the castle for generations. As children you used to sneak up there and peer at its pages, fascinated while the words and images changed before your eyes. Mostly they were just incomprehensible. Sometimes they gave you nightmares.
One legend has it that the book was written by the witch that used to live in the castle. With it she commanded animals and performed all sorts of wonders. Many locals still look back fondly on what their folklore remembers as a benevolent dictatorship, despite the official history which paints her as a sadistic power crazed lunatic.
Another even older legend suggests that the book was written by persons unknown and brought to the castle by a traveller ‘for safe keeping’.
Recently, rumors have filtered up to the castle concerning dark agents moving through the land asking for a ‘mysterious book that won’t sit still’. They keep hidden beneath their cowls, and have a strange accent. They are free with money and this buys them some loose tongues. But those that spend too long talking to them develop unpleasant twitches and have nightmares afterwards. Dogs get their hackles up when the inquisitors are near, and cats hiss fit to explode. Worse, those that are a bit too clever for their own good, or who appeared to have actually had some useful information concerning what can only be your book, have all turned up later dead and horribly mutilated.
The question: The book cannot fall into the hands of the dark agents. You know this deep in your hearts though you cannot say why. Perhaps the book itself told you.