The 3rd Era Freeport Companion
has rules for insanity and madness, as well as a Knowledge (forbidden) skill. This skill can only be gained and raised by reading forbidden tomes or otherwise being exposed to such knowledge, much like the Cthulhu Mythos skill in Call of Cthulhu
. Acquiring Insanity Points is detrimental to your Wisdom score, which will affect future Will saves to resist gaining more of them.
As to how dangerous it is to possess such a book without reading it, I'd say that depends on whether the book is magical or not. A nonmagical book of forbidden knowledge can be disposed of just as any other book can be. In Lovecraft's work, most copies of the Necronomicon and similar books have been destroyed by those opposed to the cults detailed within them. In the Call of Cthulhu
RPG, these tomes also serve as ways to learn spells, but even a D&D spellbook is easy to destroy unless it has been magically warded in some way.
On the other hand, if you want some forbidden tomes to be dangerous even to possess, then treat them as cursed items. Choose drawbacks such as the following from the DMG's list, or make up your own:
- Character becomes selfishly possessive of the item.
- Character becomes paranoid about losing the item.
- Character gains one negative level.
- Character must make a Will save each day or take 1 point of (Int/Wis/Cha) damage.
Note that ability damage heals at the rate of 1 point per day, so that last drawback is more annoying than deadly. If you're using the Freeport Companion
's insanity rules, I'd suggest a failed Will save gains the character 1 Insanity Point. And as magic items, cursed books are allowed saving throws even when unattended.
To make a forbidden tome truly dangerous, along the lines of The Evil Dead
's Necronomicon, make it an intelligent cursed item. Assign powers that fit the book's theme, such as deathwatch
, Knowledge ranks, cause fear
, etc. These books are almost always evil, with the other alignment component determined by the book's content or origin (LE for diabolic lore or texts in Infernal, CE for Abyssal or lore about demons or The Unspeakable One). The book will continually test its Ego against its owner's will, with success forcing the owner to open and read the book, or in extreme cases, even read aloud one of the rituals it contains.
In most campaigns, the majority of books containing forbidden knowledge should be normal, nonmagical books, a few of which may also be spellbooks with a handful of spells at most. Cursed books should be rare, and intelligent ones even more so, or else they'll lose their mystique in your campaign, and the characters will learn that it's safest to simply destroy any such book unread. And where's the fun in that?