I see many posts on here, discussing how to handle players and spells, investigators and "Mythos Magic", how a Keeper should handle players who want to delve into the Forbidden Arts and so on. I'd like to put forward my own take on this subject, discuss it and leave options 'revealed' for Keepers to use in future games.
Before I start, I must declare certain, ah, strictures:
This is game use in long campaigns; considering how long it takes to digest a Mythos tome, possibly benefitting from knowledge of spells, one-shot or single-session scenarios needn't worry. Unless, of course, they become campaigns so Keepers should keep some ideas to the back of the mind.
Campaigns where the Investigators are intended to become "the bad guys" or, at least, rival cultists needn't use any or all of these considerations.
You do you! Experienced Keepers know that no matter how well planned, how careful the intent is in a game development, we're down to a random factor; not dice, but players. If players choose to change the dynamic of a campaign, then so be it!
These are my own thoughts, in-house rules and experience-based considerations. They are in no way, shape or form official guidance or rock-solid legislation! Pick what you like, ignore what you like. I'm happy to discuss my "considerations" and, of course, completely accept a good argument for difference. These are mere, um, ramblings.
Moving on ...
Consideration 1: Vive Le Difference!
Call of Cthulhu is not a fantasy role-playing game! It is a role-playing game, and it has fantasy (i.e. unreal) elements to it. It's prime element is being based on HPL's literature. Thus at its core, the Investigators are the Good Guys and the Cultists and various nasty creatures, the bad guys. It is based in reality - it doesn't matter what period of history - but that reality accepts magic or at least occult practices are real! However, in this Game Reality, Investigators are not just fantasy characters in sharp, 1920's suits. This is why (in my own experience) CoC stood out from the games of the day. RuneQuest gave us a good alternative system; Call of Cthulhu gave us a game based in (a) reality!
We hit the double-edged athame straight away: it's reality but it allows for Mythos and non-existent opponents with magical skills! So why should an Investigator not learn spells? Why not indeed?
Because investigators - player characters - are the ordinary hero. The guys and gals who are horrified by magic and it's potential for corruption. They are not superheroes, they are the "reality" person in an unreal situation. And, getting back to my proposition that the 'core' value of Call of Cthulhu as a game is that you play an ordinary Joe/Jane in an extraordinary world. And having a list of spells you can cast is hardly 'ordinary'! Not to be too scathing but we're playing CoC and not some D&D-clone; no character classes, no hit point to experience correlation, no "fight first and worry about the law later" thinking.
Well, the latter might still work but you know what I mean!
Consideration 2: Rarity.
Taking that magic exists in the 'reality' of the CoC game, it's not acceptable! It's not the 'done thing'. It's been frowned upon by society as soon as one bloke-dressed-in-sober-clothes said "That woman gave my cow a nasty look and now it's a bad milker - is there a connection?"
Magic may exists but firstly, it's not common, and secondly, it's open use is frowned upon. In so-called Enlightened Times, it's dismissed as arrant nonsense. So, if you were in the know, and had such abilities at your command, then you wouldn't wander around looking like a star-spangled Gandalf or even "that weird guy who talks to his furniture and knows what underwear my Mom has on!"
The argument that 'experienced' investigators understand that magic has a reality is fine ... but how does that sound to those not in on the secret?
Point is, magic in Call of Cthulhu exists but isn't common. In fact, it's so rare it acts as a warning. After all, who casts spells but those linked to the Mythos? Aren't they the Bad Guys?
Consideration 3: Usage.
In this I appeal to Keepers with a personal 'house rule' that's worked most of the time*.
When an Investigator gets their hands on a major spell-giving Mythos book, and they fail to comprehend a spell (i.e. learn its use) then the Sanity check is made (and failed) then they cannot ever learn that spell! We're not talking about someone screwing up an exam, that they can retake later, using another book. They just "don't get it!" They have experience of that spell, and may recognise it's casting or components that are used but it's just beyond them!
This may discourage players who decide their Investigators wish to embark on a career as being an alternative Doctor Strange (TM) and completely miss the point about Call of Cthulhu (see Consideration 1).
* Right up until I had a player who'd decided her investigator who'd 'accidentally' discovered his occult powers and 'collected' spells for future reference. When they did actually learn a spell, it blasted their sanity. It wasn't only realising the 'truth' but the 'true' believer was shocked to find they had been right all along! Zen and the art of Magic, eh?
Consideration 4: A Matter of Trust.
If you are a group of Investigators, who've been through many horrors together and trials yet come through, defeating those damnable cultists, how comfortable do you feel (in 'game reality') when the professor starts to show an ... eagerness to acquire arcane books, blood yet to dry on the covers, to *ahem* add to his knowledge in the fight against etc. etc.? It may be simplistic but "Cultists use magic so using magic may make you a cultist" is a thought for the characters. Yes, you might be gathering counter-weapons but, then again, you seem a bit eager to obtain such-and-such ingredient or be a tad careless over learning that a child has been kidnapped: "Oh, yeah. It might mean they're going to perform the Rite of Akerchaly. It's almost May Eve? We'd better get a move on then ..." A Keeper must take team dynamics into consideration. Should everyone in the group be okay with one person becoming the Level 7 Mage (in CoC terms) then so be it. See what happens. But isn't this actually going against the feel of the whole game?
Consideration 5: Balance.
Many folks - Keepers and players - are okay with the prevalence of magic, reasoning that the bad guys and (let's face it) the feckin' creatures of Call of Cthulhu are so strong that the Investigators need spells, to fight the Mythos on it's own terms. I suggest it's not a form of arms race. The game itself is balanced.
I know, I know - settle down and hear me out.
The Investigators fight the Creatures because they don't realise how incredibly nasty they are. We, as players, do. So if you decide you need to weapon-up, you - not your character - is thinking "I'm in deep doo-doo here!" So, as time goes on, and your characters realises magic is 'real' then you decide to 'arm' your team with some arcane tools. Such as learned spells. These come at a cost. Not only to Magic Points but to your sanity. To balance your strength against Mythos, you are willing to lose your mind! This expects us, as players, to role-play! Worrying about the consequences to your mind, being reluctant to fight fire with fire, is role-playing. Just saying "Yeah, whatever; what dice do I roll to subtract sanity?" is not.
Call of Cthulhu is based on the stories of HPL, one of which is "The Dunwich Horror". Three university professors versus a profane avatar of Yog-Sothoth! Physically, the challenge seems unbalanced but, heck, they can make "Library Rolls" and such. They do, one of which gains the knowledge - and spells - to remove the creature ... hopefully. The head boffin bags "The Powder of Ibn-Gazi" creation and "Dismiss Deity" spell! Cool! No. Not really. The lead-up research almost killed a very intelligent professor and the performance required three intelligent, and scared, people!
If you want to play Call of Cthulhu, keep in mind that magic happens ... but it comes at an awful cost. Far more than a few points off your character sheet!