The Tome of Eldritch Lore is a horror trope. The tome is a book which contains forbidden and dark knowledge which is likely to drive the reader insane and cause all sorts of other detrimental side effects as well - even if no one is actively reading it in some cases.
The ur-example is probably H. P. Lovecraft's Necronomicon - although the idea of the grimoire is far older. Lovecraft's creation was specifically of the "repository of things Man was not meant to know", but other examples could well include books of spells and lists of demon names. Indeed, even without Lovecraft's context, the direct translation of Necronomicon as "book of the names of the dead" suggests that it might be a useful tool of necromancy.
More fantastical examples can turn out to be the laboratory notebooks of one or more wizards, know for their groundbreaking research. Even if these wizards were positively reputed, such knowledge - especially where "experimental write-ups" are incomplete or fail to mention (or explain) the "standard safety precautions", this can still be dangerous. The edgier the wizard, the dodgier the research. Also, expect such things to be far from clear - in environments without IP law and/or where people are worried about their work being stolen and/or misused - expect a great deal of allusion, metaphor and outright encoding.
"Whiter" versions of this book may be, effectively, magical textbooks - explanatory texts setting forth points of thaumatology for the benefit of students. The context in which such a thing would be written bears careful consideration1.
- Artifact Of Doom
- Book Of Shadows
- Codex Gigas
- Scanned Multi-Alphabet Arabic Manuscript Online
Game and Story Use
- Mandatory for Cosmic Horror campaigns.
- In settings where invocation - and outright summon magic - are popular, the names of spirits and demons, used as part of summoning and binding rituals, are prone to be a significant currency amongst users. Obviously, such knowledge is unlikely to be published, but each worker might keep their own registry - probably including any other information on the entities in question, records of successful (and unsuccessful) summonings and, if applicable, notes on preferred payments and bargains. Such a registry that comprised a lifetime's records from someone known to have controlled a great many spirits might be a significant prize.
- Cost/benefit for use of these things should depend upon setting - horrific secrets about the true nature of reality may be more harmful (and less use) that a demon summoner's journal (although in a wainscot fantasy setting, such a thing might be unnerving even if the reader didn't actually believe in the supernatural.