In established RPG campaigns, it can be fun and interesting to shake things up from time to time by throwing your players a curveball.
- The PCs kill a dragon but are surprised to find the residents of the valley chase them off because the dragon was protecting them from predation by nearby tribes of orcs.
- The PCs recover a holy relic for the local magistrate; later, they learn that the magistrate was secretly a worshipper of Asmodeus, who steals the relic for her fellow acolytes and implicates the PCs in the theft.
- A trusted NPC ally or mentor has been possessed and lures the PCs into a trap and tries to kill them.
Subverting player expectations can carry a cost
A common complaint heard from DMs is that their players don't like to roleplay, but if you undermine their achievements or take away NPC allies and hard-won rewards in a seemingly arbitrary manner, your players will be less inclined to invest gaming time in developing those relationships.
D&D—and to an extent, most tabletop RPGs—treats the DM as the portal through which the players experience and interact with the world. As the DM, you provide exposition that describes locations, people, and monsters; you determine the actions and reactions of everything the PCs interact with. As a result, it falls to you to make the world behave in a internally consistent manner.
Establish up front what elements of your game will deviate from the norm
The Eberron campaign setting is a good example of providing a novel experience for established RPG players. The Eberron Campaign Setting sourcebook for D&D 3rd Edition lays out new many ground rules for the setting, among them:
- The nation of Karrnath resorted to using undead soldiers in the Last War, and powerful undead still hold positions of authority in the government.
- The goblinoid races are not always bands of scavengers or thugs, but heirs to the proud cultural and militaristic traditions of the fallen Dhakaan Empire.
- The nation of Droaam is home to many monstrous races (including ogres, medusae, hags, minotaurs, and gargoyles) and their citizens and ambassadors are common in the city of Sharn.
- Half-orcs and half-elves are true-breeding races, although they can still result as offspring from mixed-race parents.
Consider writing up a campaign reference for your game to explain what changes your players need to know about in order to play their characters effectively. Are certain classes or races rare or unavailable? If your players know that elves are a near-extinct race because their kingdoms were razed centuries ago, they can react appropriately when being introduced to an elvish NPC.
Foreshadowing can add richness and depth to your game. I was in a D&D session where the party encountered the skeleton of a giant toad partially covered in green slime; upon investigation we found green slime marks on the ceiling. Later, one of the PCs was nearly killed by a green slime that dropped from the ceiling. Far from being arbitrary, the first encounter told us what kind of creatures we could expect and how we might be attacked.
Consider what details you can sprinkle into your game that can be pieced together—perhaps after it's too late! Does the magistrate collect the writings of worshippers of Asmodeus to "keep them away from impressionable minds"? Do some of the inhabitants of the valley change the subject or claim ignorance when asked about the whereabouts or habits of a marauding dragon?
Another way to foreshadow a twist is to tie the PCs' actions into the intended twist. Was the magistrate keeping a low profile until she heard the PCs boasting about wiping out a conclave of cultists of Asmodeus? Do the PCs overhear some Orc marauders warning their fellows to "avoid the valley of that cursed flying snake"?