There are many ways to frame that sacrifice. Pelgrane's Trail of Cthulhu: Bookhounds of London setting offers the opportunity to become a trader in the grimoires of the Mythos. The protagonists are booksellers who sell and dicker for those same Mythos tomes, trying to make their fortune by trading in damnation. Ken Hite, the author, describes it as a game about "the skeevy people who sell grimoires to people who want to end the world. Yeah, it's the wrong thing to do, but you have to make rent." It's possibly the only RPG where shelfwear and foxing can be more catastrophic than a broadsword to the face.
Here the grimoires change the narrative by being the focal point of the character's day job. Before, it was a simple exchange: Sanity for knowledge. Here, the question is more complex. The protagonists don't have to sacrifice their own Sanity. They never have to crack open a single book, if they choose not to. However, their day job brings them into contact with people who cheerfully trade off Sanity for knowledge, and who aren't the sort to be trusted with occult wisdom. Now the narrative becomes, how close to the line will you go? Your livelihood depends on doing business with these madmen. If you take the high road and refuse to sell, they may become desperate and do something rash. Or you could make a profit and damn the consequences; not very high-minded, but bills don't pay themselves. You could even go the sneaky route and fake a tome, making a tidy profit while at the same time hopefully delaying the end of the world, assuming that the customer doesn't catch you at it.
Yet all the while, at the root of the campaign are those same grimoires, brimming with twisted wisdom. The greater narrative is focused squarely on them, but this time the issue isn't knowledge in itself. The question is what to do when your personal desires for money, success, and so on, conflict with the greater good. That narrative could vanish down any number of intriguing rabbit holes, such as: What if the buyer represents the greater good? They may use their knowledge to help save rather than destroy the world. Or, what if you choose not to sell but a rival also has the same item for sale? Do you meddle in their business too, or stay silent?
In myth, the grimoire promises its owner magical power. In fiction, the grimoire also provides a kind of magic, as authors like Lovecraft use it to create artificial myths powerful enough that even today some people are convinced a real Necronomicon exists. However, it's in gaming that grimoires provide their best kind of magic: Forcing players to consider the consequences of their actions. All the while the narrative flows on, with the grimoires acting as vital plot points. They provide clues, knowledge, and mystic power to their owners.
Of course, there's a price ...
Adam has written for Chaosium, Pagan Publishing, Miskatonic River Press and Pelgrane. A new Pelgrane project is in the works!