I recently discussed flim flam artists and fakers in Trail of Cthulhu, and suggested that in any setting there ought to be at least some fakery. Without false knowledge, players will come to expect that everything in front of them is the exact truth, that if early indications are that vampires (say) are at the root of the scenario then they won't go looking for other explanations. I used the example of a psychic healer in that post, but today I want to talk about something slightly different: false knowledge, specifically false Mythos knowledge.
Now, before I go any further on this topic I need to make one thing clear. If the Keeper intends to use false Mythos knowledge in a campaign, I strongly advise that he discusses this with the players before the campaign begins. The Keeper must always be perceived as a fair arbiter, whether of rules or anything else to do with the game. Introducing house rules without telling anyone, only bringing it up when a player falls foul of the house rule, is the quickest way to ruin gaming friendships I can think of. Bear this in mind!
Cthulhu Mythos is defined in the main book as the secret rules of the real world, [not] the ignorant scrim of physics and religion. You recognize the great names, and the truths they conceal ... The primary use of this ability in the course of an investigation is to “put together the pieces” and draw upon the terrible knowledge that you have been subconsciously suppressing, achieving a horrific epiphany. Using it costs both Stability and Sanity, and the benefit provided is to point the characters in the right direction towards solving the scenario's mystery.
If there is such a thing as false Mythos knowledge, then based on this description two things are clear: first, it cannot - by definition - point the characters in the right direction. Fake is fake. They may accidentally stumble in the right direction regardless, but that's their good luck. Second, though it might cost Stability to use it, false Mythos ought not to cost Sanity, since - again, by definition - false truths do not reveal great names or any significant information, and it is that information which wears away at a person's Sanity.
Therefore false Mythos does this: it costs precisely the same, in terms of Stability and possibly also Sanity, as a real Mythos reveal of the same magnitude - as discussed on pages 74-6 of the main book - but it provides no actual insight. This may mean that the Keeper has to come up with a completely false lead for the players to follow, or that they are presented with a means of solving whatever problem faces them but that this method will not actually work when used. This can be especially devastating should the players be basing their entire last-ditch plan of attack on false knowledge. The results could be severe, possibly even death for one or more characters. The Keeper needs to factor this possibility in when including false knowledge; only if the Keeper is happy with the potential result - up to and including a total party kill - should the false knowledge be used.
There is an obvious flaw which you probably spotted straight away. If false Mythos costs no Sanity to use, then players will immediately spot it for what it is. Yes ... but there are two points to consider. First, that by the time this becomes apparent the false Mythos has been used and therefore has done all the damage it can do. Thus imposing a Sanity penalty on top of everything else could be seen as overkill. Second, that if the Keeper wants to maintain the mystery there are rules already in the game to help achieve this. The Denial rule (p 75) states that if, at the end of the adventure, there is absolutely no proof of your horrible experience – samples, photographs, recordings, eerie artifacts – then your Sanity rating recovers by 1 point. Effectively it "never happened," so the penalty is reduced. In the case of fake Mythos, it can't ever happen - it's fake, after all - and in that instance I would recommend that the Sanity rating recovers by the amount "lost," whatever it may have been, at the scenario's conclusion, rather than just by 1 point. It could be argued, particularly in a Purist setting, that 1 point is all the player is entitled to regain, but that seems harsh to me; again, if the Keeper intends to do that, I strongly urge that this be made clear at the outset.
Why use fake Mythos? Again, this goes back to the flim-flam argument, but I'd go one further and say this: particularly in a setting like Bookhounds of London, which depends so much for its impact on fraud, deception and vice, it seems counter-productive to suppose that every source of Mythos knowledge is telling the truth. Some of those dusty grimoires must be fakes, but how then to separate those - from a rules point of view - from the ones that aren't?
It's not as if there have never been frauds in real life. The Vinland Map, the Book of the Zohar, Jack the Ripper's Diary, to name just three forgeries created either with the deliberate intent to deceive for money's sake or as part of a more insidious attempt to spread false doctrine. Moreover there have been any number of occasions when a "new" work by a famous literary figure has suddenly been discovered; any number of artists who have lived off of their ability to copy a famous person's style. Given that, it is increasingly likely that someone - probably quite a few someones - have tried to grift their way to fame and fortune by copying, say, the artistic flourishes of a Pickman, creating a play in the style of The King in Yellow, or discovering a hitherto unknown von Juntz. These wouldn't necessarily have been modern fraudsters either; scarcity bumps up price, and as soon as the Cultes des Ghouls went on the bonfire in the 18th century there would have been someone trying to persuade one of the mystical societies - perhaps a Hell Fire Club - that they had access to one of the Comte d'Erlette's unpublished works, and was prepared to part with it for a small consideration. Frauds of this type would be all the more difficult for a modern researcher to identify as frauds, since they would have been created with materials appropriate to the period.
The biggest difficulty with using false Mythos is that it requires bookkeeping. Each and every source of Mythos would need to be identified and cataloged; you can't just add them straight into the pool with no record of where it came from, or you'll never be able to tell the good stuff from the bad. The player will need to record that the character has, say, a Cthulhu Mythos pool of 3 points, of which 1 comes from direct experience during a scenario (and is therefore true), 1 comes from studying a text (which may or may not be true), and 1 comes from miscellaneous occult researches (talking with ghouls in the churchyard, or otherwise learning from primary sources, which is therefore also true). Anything based on direct, personal experience - an encounter, consulting with Mythos entities and the like - is certain to be true. Nothing else should be as certain, so in those cases where a Mythos point has to be spent the player needs to specify where the point came from. That should make for some very tense decision-making moments, as pools start to deplete and the players begin to wonder: ought I really trust the information in that tome? After all, I got it from Bainbridge, and he's a born liar if ever there was one ... but he might not have been wrong about this.
Naturally false Mythos should only be used at the Keeper's discretion, and with full knowledge of the probable outcome: defeat for the characters, and therefore the players. That doesn't have to be a bad thing; this is horror gaming after all, and players are bound to lose from time to time. However don't forget the advice I gave earlier; the Keeper should always be seen as a fair arbiter of events, not as someone who changes the rules to suit themselves mid-session. It should also be possible for the players to detect frauds before they do damage, and there are skills - Bibliography, Textual Analysis, Document Analysis - that can help careful players do exactly that.
Provided of course that they remember to use them; after all, even if the cult leader whose group they just defeated prized that tome above all others in his collection, his belief doesn't make the Mythos knowledge contained therein true ...