Early in our Warhammer Fantasy RP campaign, the lone human in the party (Gunter Von Graf) ended up with a heretical tome I did not expect him to find or keep after the first couple adventures. Now, when the PCs decide to actually keep the tome you were using as a plot-moving Macguffin, and not turn it into the authorities, you could say: "The tome talks about blasphemous stuff and is bad to read. Burn it or give it to the witch hunters," or you can tempt them with knowledge and power...
So I realized this was a perfect opportunity to Lovecraft it up!
We were playing through the "Eye for an Eye" adventure from the main WFRP 3e, and in the library of Grunwald Lodge (a large manor/fort in the woods north of Ubersreik) there were two outlawed books tucked away forgotten in the library, whose purpose was to both tip off the PCs that not everything was as it seemed in the lodge, and the cult leader would use them in the final summoning ceremony to beef up the demon. Of course, finding them was pretty difficult (3 purple challenge dice), and required an Education check, which is a skill only two of the first-level PCs had, so I didn't put too much thought into the library at the time with all the other prep work required for that adventure. Instead, I had spent time working on making up iconic accents for the evil doctor and his assistant, and assuming the PCs would catch on to the guards being poisoned by the doctor and generally finding all the creepy clues in the hospice. Of course, the PCs are going to come to completely different conclusions than you expect, and focus on stuff you never even considered.
In this case two of the dwarves decided that the library had to be the source of the evil, mostly because they couldn't read, and the best way to search the library without tipping off the librarian (who they just assumed would be a cultist if anyone was) was to distract the librarian by getting into a fake brawl with eachother. While Otto (the librarian) tried to shoo the illiterate dwarves out of the library, Gunter searched the books with the help of the one remotely educated dwarf and sure enough, he barely rolled a success on the Hard Education check. So he found one of the books. He pocketed the book in his cloak while the more physically-proficient dwarves knocked eachother over a bookcase. This got them permanently banned from the library and didn't stop the demon summoning or anything, but it did put a heretical book in the human player's hands, which tipped them off that heresy was afoot in the lodge. Though now the player has an eldritch tome and wants to know what it says.
First, you need a sweet name. I recommend any of the classics from Lovecraft literature (outside the Necronomicon, that's too obvious), like Cultes Des Ghouls, the Pnakotic Manuscripts, or Revelations of Glaaki. In this case I chose "De Vermis Mysteriis" or Mysteries of the Worm.
Now, following the time-honored tradition that dwarves are magically dense, I deemed the book doesn't register at all when the dwarves handle it. However, when the only human handles the book, all the creepy things come out, like whispering in an unknown language, pages warming at the touch, or ending up next to him when he wakes up when he knows he stored it away in his pack (if you can find a copy, the d20 Call of Cthulhu rpg sourcebook has a great list of random tome creepyness). But occasionally when we end a session at an important point, or I know something magic-related is coming up, I'll throw together a page of the tome that might just fall open to, or he remembers from perusing it, print it out and hand it to the (human) player.
My first attempt at giving PCs some information like this worked really well. In this particular case the adventurers had just killed the cult leader but hadn't prevented the summoning, resulting in an unbound spidery demon rampaging through the fort at the end of the session.
As per the old-school Warhammer 2e rules, I had previously decided that unbound demons are unstable and have to drink souls to remain outside the Warp and remain in the Prime Material. And to make it extra creepy, while this particular demon was outside the warp it would attack people with its probiscus and try to lay as many demon eggs in people's bodies (killing them) as it could while it was in the physical realm. The eggs wouldn't hatch or anything, but the players and the demon didn't know any better. It just seemed like a horrible way to die.
Anyway, I put together a six-piece tracking marker and figured that for every turn the demon doesn't kill someone the tracking marker will move one step towards dismissal. All the demons in the Warhammer book are WAY more powerful than any members of the party, and if it were to stay in melee with anyone for more than a turn they were probably going to die.
Of course I couldn't just tell the players any of this, all they see is a demon materializing on an evil altar. So I slapped together the following tome page using an online tapestry generator between sessions, printed it out, and told Gunter's player one of the pages from the book he saw earlier just slithered its way into his memory at the beginning of the next session:
It goes without saying that rhyming is always a must with eldritch tomes.
The sweet Historical Tapestry generator I used for pictures like the above is available online here (sound warning: the website plays a little tune while it loads). You can drag pictures around and write stuff to make a dark ages-style tapestry. In this case the player figured it out almost immediately and informed the dwarves to keep away from the demon. But between house guards, cultists, the PCs, and a horde of rampaging beastmen attracted to the chaos ceremony, the demon had plenty of targets. This fear of allowing the demon to 'recharge' actually led to the PCs pre-emptively shooting people that were in the demon's clutches rather than letting the demon have them. This strategy was of course secondary to setting the entire place on fire and burning down the fort around the demon (and cultists), but both were sound strategies. Mercy killings and witch-burnings. You really can't get more grim darkness of the Old World than that!
Of course, since then I've tried to use the "eldritch tome page" maneuver to get the PCs to do what I want, but since then Gunter's player has become more and more paranoid about the book, and is sure I'm trying to get him possessed or killed by witch hunters, so won't follow the page's advice even when he understands the riddle perfectly. Still, its fun to let the players know what the tome wants them to do, and let them decide whether its being helpful or manipulative. So far their paranoia has made them sure its being manipulative (even if I'm actually trying to help them!), much to my delight.
Oh well, that's life in the Old World!