To be upfront I am not talking about the Great Old Ones or the Outer Gods in the game, I am making a statement about the levels of power in the game, and when and how you should cross certain lines as a Keeper and scenario designer.
A threat of some kind has to exist in any scenario, scenarios are stories and stories are based on conflict. No conflict, no story. Call of Cthulhu is not, however, simply about combat, a clash with a gangster or a cultist can be just as deadly to an experienced character as to a novice. Granted, a character who has been around a while may have some better armament in a few senses, notably spells. But a gun in the hands of a normal human opponent is just as deadly if it hits. Unlike some rpgs, a character does not gain the ranks of power that make them immune to lesser threats. And for every gain in power a character in CoC has, he has usually put his sanity at greater risk, and this is more often than not more fragile than it was earlier. Skills can grow, spells can be learned, but…
In the face of this knowledge, certain foes are an ‘even’ challenge early in the game, about as tough as a player character making an even fight an even challenge. This still leaves combat risky and challenging, and in a game world without magical healing and resurrection, smart players learn caution in any potential conflict.
These ‘even’ challenges are things like ghouls, deep ones, mi go, byakhee and the average cultist. Encoutners with small groups of these types can form combat encounters that round out the adventure, as long as they serve to further the story, and hopefully strengthen the player character’s position in their situation.
Then you begin to encounter the slightly greater challenges, leaders of small cults, lesser wizards, dimensional shamblers, and monsters that are a little tougher, little being a very relative term. We start to cross into the territory where a single encounter with a single one of these creatures can put the entire party in danger, and at great risk. Encounters with singles of these entities should be relatively major encounters, if not the climax of lesser scenarios, and largely begin to become the events player characters should try to prevent from happening if possible (stop the summoning, circumvent the wizard, banish the monster, etc).
After this we begin to encounter the much greater challenges. These are things like the Dark Young of Shub Niggurath, shoggoths, cthonians, star spawn of Cthulhu, Dholes, Lloigor, Nyarlathotep in his least forms, and greater wizards. These creatures form the climax of more major adventures, are creatures to thwart without combat if possible, or flee from.
After this we start to encounter the biggies, Cthulhu, Nyarlathotep in his lesser forms, Cthugha and the various Great Old Ones. These are definite major climaxes in the scenarios where they appear, to be avoided or circumvented if at all possible. Encounters with these are to be avoided by the players if they possibly can. If any of these actually show up, it will take luck or preparation established in the scenario for the player characters to survive these encounters, if not the risk of the end of that particular campaign world.
Above this, we have the Old Ones, entities whose mere presence should be catastrophic and almost certainly the cause of multiple deaths by player characters, and very likely the end of the campaign in a very bad way. (There are some exceptions to this, to an extent, Yog Sothoth peeks in now and then without the world ending, but generally in a few of his avatars, and when he takes an active interest in our universe, it’s bad news.)
The main reason I’m bringing up all this, is because in published scenarios, it is more than a little surprising how often you run into monsters that parties should not be able to beat, and sometimes in an incidental role. Case in point, in the book Before the Fall, the scenario “the Innsmouth Connection” has a shoggoth as a scenario topper. In the book Curse of the Cthonians, the scenario “Dark Carnival” has a plotline that hints at a shoggoth, but turns out to be a cthonian working with the general villains of the piece (as if that’s better). Dark Young, shoggoths and cthonians show up with alarming regularity, let alone tougher creatures.
My personal gaming style is to allow player madness and death as fits the story, but to avoid pointless character deaths. I do suggest caution when using established scenarios, think of the campaign you’re fitting it into, of the gameplay you and your players want.