During last night's session of my Cthulhu by Gaslight campaign, four-fifths of the characters in the party voluntarily got stoned on opium at one point, so anxious were they to ease their addled minds. And that was before one of them woke up to having his teeth pulled out by a clutch of charcoal imps (poor Dr. Hume is now missing four chompers from his grill) and another character nearly had his eyes clawed out and was left with terrible facial scars.
Reflecting after the session, I couldn't help but smile. My campaign had hit the Cthulhu "sweet spot."
Every RPG has them, and they're all different. It's what makes the RPG campaign such an enjoyable format of gameplay. (And why I don't particularly care for systems that try to spread the sweet spot over the entire course of play.) Initially, the party is still coalescing, the players are still getting a feel for the world and the way their characters interact with it, and larger themes or arcs have yet to organically emerge. Then, all of a sudden, a switch is flipped and the whole tenor of the campaign changes. You're playing D&D and suddenly you can mow through goblins and orcs with aplomb or successfully plumb that dungeon level that's been so deadly up til now. You start thinking about tracking down those rumors of a nearby dragon's lair. Or you're playing Call of Cthulhu and all of a sudden everyone is paranoid, cynical, and (in my campaigns at least) mutilated in some way, either mentally or physically. These two sweet spot examples couldn't be fixed at more extreme polarities, yet they are equally thrilling once you reach them.
Of course, eventually you pass out of the sweet spot too. The half-ogre fighter with the magical zweihander starts soloing remorhazes. Nothing feels challenging anymore. Or else your intrepid investigator is just too run down, too used up, to be of much use and everyone's running around with Cthulhu Mythos scores well into the double digits. It's just not quite the same once you've read the Necronomicon cover to cover or joined the Round Table.
But by that point, you're already thinking about starting it all over again...