I want to have a series of very hard to understand tomes (à la the Necronomicon) in my D&D 3.5 campaign. How do I model their difficulty? What kind of checks should be used so that players have an easy time understanding the first few tomes, but need to have demigod-like insight to read the last few?
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Skill Based Approach
Decipher Script is the skill that would be used to read and understand something like this. You can set the DC to varying difficulties depending on the difficulty of the material. The rules recommend "30 or higher for intricate, exotic, or very old writing."
For extremely obscure stuff, you could also require a relevant Knowledge or other skill check, to see if the character actually understands the most difficult material. For example, Decipher Script tells you that the material is some kind of ancient incantation, but a DC 25 Knowledge - Religion can tell you that it's a prayer to a god from a dead religion, and Knowledge - History can tell you that said god's followers were wiped out in a cataclyscm 374 years ago after a magical accident, and Spellcraft tells you how to recreate whatever spell the incantation is meant to do.
Or whatever details you want to plug in. You can make it extremely difficult and require several players skillsets working together (or talking to NPCs) to decipher it if you chain enough skill checks together.
RP Based Approach
Another approach is to create a puzzle the players have to actually solve themselves, rather than using dice. Write the material and then convert it to a symbology (runes, characters, heiroglyphs, etc).
You can also write the whole thing in riddles, poetically, or metaphorically, but it's a lot of work and possible to write something that nobody can actually solve. Those kinds of puzzles don't use any game mechanics to solve. You hand it to the players and let them try to do it.
You can augment this by having some pages be damaged and some words missing, giving them pages out of order, and letting them find them gradually over time.
What is the tension here?
OK, so your players have a tome. It’s hard to understand, so they’ll presumably need to use Decipher Script, Knowledge (arcana), and most likely various other related knowledges (religion, the planes, history all seem pretty likely).
Where can they fail here? If they roll the dice, and roll low, what does that cost them? If the answer is just “some time,” then you shouldn’t really be trying to roll dice on this at all; just let them say “well, we’re going to spend as long as it takes to figure out everything we can from this tome,” and then you go “it takes days, but you figure out X, Y, and Z.” No rolls, because it doesn’t matter. Maybe you look at their skill bonuses and use 20 + the bonus to figure out the max they could have gotten, whatever.
Or make it matter. If they have a deadline, losing time can matter, though be careful with that – if they don’t literally have a “clock” visible in front of them, this tends to fail pretty flat. If they know they have three days to do things, you could roll three times, once for each day – and each day they are provided an opportunity to stop studying and go start implementing whatever preparations they can based on what they’ve learned. But if you have absolutely no reason to suspect they’re going to do anything but keep rolling, just elide all those rolls together in “taking-20.”
Because ultimately, that’s what the game is about: making decisions. Maybe they worry about the deleterious effects of reading a forbidden tome, and so there’s a decision there: do they read the book or not? Once they have decided to read the book, or not, they probably aren’t going to change their minds unless somethings changes: they find what they read to be more disturbing than they expected, or they get more desperate with time running out so they decide to turn to the book. But if they’ve already opened the book, and they’re learning things, and there’s no reason for them to stop, assume they won’t, and let them get as much as they possibly can out of it.
From there, by skipping the “let’s roll until we roll high enough,” step, you can build out opportunities for them to find more information and resources. You can let them do Gather Information to find ways to access the private collection at the arcane library, discover a crazed hermit who has been touched by the prophecy, or track down some of the cultists responsible to try to get more information. You give them options and let them make decisions.
Try the Decipher Script skill for interpreting the books' contents. Start with fairly easy DCs and increase them as you get harder and harder books. Tell your players to expect Decipher Script checks, because it's not one people commonly invest in.
If the books contain terrible knowledge which may have effects on the player, consider having them make Will saves on successfully reading a passage to avoid or mitigate ill effects.
Decipher Script, Wisdom Damage, and Increasing Madness are all your friends here.
Decipher Script is a skill that few people bother with. Having a set of DCs that start at level that just intelligent characters can make without investing in the skill (15, 20) and then moving on to the kind of DCs that characters with full ranks will need to take 20 on (35, 40) will do the trick.
Then, have them start taking wisdom damage (driving them insane) the longer they spend reading (so take 20 will have a serious cost).
Then refer to a list of derangements (there are many on the internet, or there are some presented in the 3.5e 'Book of Vile Darkness') and start making players that read too long go insane.
In addition to the various encounters springing from them contacting the forces of darkness by reading the book, whole inns sinking into forgotten realms as the party flees, terrifying aberrations haunting and stalking the party, etc.
Of course, for all this effort, you'll have to make it worthwhile for the party to continue doing it. Extra power, or plot-necessity, is advised.
"They're Those Kinds of Books..."
The SRD contains rules for Sanity. It might be worthwhile--if you expect these texts to have a major impact on the campaign--to use the sanity rules and the Knowledge skill Knowledge (forbidden lore) in place of Decipher Script skill checks to discern the contents of these disturbing tomes.
Real world cypher backed up with "adventure events" to uncoil the onion. Layer 0 is on everything, higher layers add more complexity. Players get actual document, and maybe updates when they crack a layer (in-game).
Easier text uses fewer layers, and the "adventure events" are easier.
Some "adventure events" should be tied to skill checks, others should be adventure hooks. Succeeding at a skill check may give you an easier or more correct route to the information you seek, failure may result in the bad guys beating you to a piece of the puzzle and in-world consequences as they exploit their superior knowledge.
The "pieces" should both help decode the tomes, and be usable by the bad guys to power up their weapon of ultimate power (the core of which the players do not have access to, and relies on unmentionable/unavailable to PCs core).
Clues to the next step should be in the books themselves, possibly revealed by peeling a layer away.
The 2 deepest books should have some actual nonsense in them.
Penultimate book: Layer 1 takes what seems to be sentences that make sense and produces sentences that make no sense, unlike prior books. Layer 0 gives some clues.
Ultimate book: Layer 1 does the same thing (makes it make less sense). Layer 0 cracks on the ultimate book is 97% more cryptic mumbo-jumbo, with one or two actual clues.
The point of this is such a thing could be a cool adventure hook. By mixing out-of-game cryptography and figurative speech with in-game skill checks and adventures, we can provide a really neat experience. And as some of those stages can be defeated without everything from in-game, the players can bypass parts of your adventure plans if they are clever enough.