A couple days ago a friend of mine recalled a conversation we had in the '10s or something, about how, if you want to run a game of lovecraftian horror, you should stay away from Lovecraft.
The idea is not original, and probably came from some blog that we read at the time. Let me expand on that.
Running a horror campaign (whatever ruleset you use) with Deep Ones, Mi-Go, Shoggoths, and the other canonical creatures and gods of the Cthulhu Mythos, is not going to make your players experience lovecraftian horror at all. All those entities have become pop culture icons, so widely known and, in the nerdsphere collective imagination, they are domesticated to the point of becoming the subjects for plushies, funkopops, cool t-shirts, mugs, political satire ("Vote Cthulhu"), board games and comical stories.
They are, quite literally, a comfort zone, which is quite literally the opposite of lovecraftian horror.
If you achieve to have your players experience even just a bit of lovecraftian horror with such set pieces, you probably are a great GM running a great scenario.
The point is, the canonical pieces of the Cthulhu Mythos can't surprise. Cosmic dread, madness, and the horror of the unknowable and incomprehensible is gone.
The only way to truly make an attempt is to create your own Mythos. Your unique creatures, entities, gods, and books. Or even better, no creatures, gods and books at all. Go for unprecedented, or at least less abused, categories. One of the points of lovecraftian horror should be: facing something that can't be fully understood, and being able to categorize it is a big step towards understanding... The other points should be: something that threatens you, and that the more you understand it, the more it questions your understanding of reality itself.
I know, it sounds daunting! Even if what you come up with is a half-botched attempt, it still has much better chances of disturbing your players' imagination a little bit.
And potentially, there's more. If you propose your group to play Call of Cthulhu, Trail of Cthulhu, Tiny Cthulhu, The Cthulhu Hack, Realms of Cthulhu, Cthulhu in Space, Cyber-Cthulhu (and the list could go on and on AND ON), well, you've already lost them. You are already telling them "We are going to play a game with the cliche of cosmic dread, madness, and the horror of the unknowable and incomprehensible, so play along".
And they might. I mean it will be a game of lovecraftian horror, except the horror.
What to do in order to pull it? In theory, the best way would be to let that horror seep into something else, just like it crawled into the lives of Lovecraft's protagonists without a warning. Let those themes (and your own cosmic Mythos) creep into another game. In theory, the further away the starting setting is from lovecraftian horror, the more impactful it becomes.
That's what they did with True Detective season 1 or, to a lesser degree, with Archive 81, or Annihilation.
But is that ok, ultimately, with RPGs? There is a final, and not trivial, conundrum.
In my opinion, players should not be sold an epic fantasy campaign, only to see it slowly become a horror campaign. Not fair at all. They signed in for A, not for B. They might hate it, and lose trust in you as a GM. The campaign might feature themes and topics some people don't want to see in a game: body horror, mental illness, religions, you name it. Not fair at all to have those things dropped on players who expected to play college drama, space opera, western action, or whatever.
So what? Probably, the best way is to inject lovecraftian horror into games that already are supposed to be horror, but belong into very different horror niches. Zombie apocalypse games. Vampires. Gothic horror. Heck, even most Sword & Sorcery rpgs might do. You'll avoid pulling a very bad trick to your players, but may still have a chance to go lovecraftian. An Apocalypse World game starting with zombies and slowly turning cosmic horror sounds like a great campaign to me.
So am I saying all those Cthulhu-branded games are worthless?
Naaa. They are fine, good games in their own way. Just know that horror, the way Lovecraft tried to convey it, will hardly be part of your campaign. You're still in for a lot of fun, with so many great adventures and campaigns available.
I'm absolutely in favor of playing within one's comfort zone. I even enjoy vanilla fantasy and will probably write a post about its merits soon!
I think most of the times gamers just want to have fun, not to experience negative emotions such as actual fear, inquietude, uneasiness, doubt. Let alone questioning their understanding of life! Sometimes it's fun to just blast your shotgun into a Deep One's face, and send the inhuman temple into orbit via dynamite... or end up squished to bits or locked at the asylum for trying.