BUT I THOUGHT WE WERE ON THE SAME SIDE?!
A long time ago, I took my party into The Friendly Arms in the original Baldur’s Gate game. I saved the game and then launched a fireball into the common room of the place just to see what would happen. If you tried that –and who didn’t?- you recall that the local constabulary contingent of the Flaming Fist teleported into the room and proceeded to destroy your party in mere moments. They were justifiably ruthless, will-trained, and probably worked well as a team. It’s hard to remember because I was quickly staring at piles of clothes and items.
Point being the PCs are not the biggest, baddest bunch in your world. Far from it. More often than not they are transient employees of a distant concern in town to kick up trouble and cash in with little regard to the local populace. Surely they don’t want to cause trouble, but it’s sure to follow. And as powerful as they may be, there are certainly local concerns that you would be encouraged to make an appearance or two to remind them of the fact.
The PCs may be a potent force of good, and powerful in their own right, but their swordsmanship, sorcery and stealth are no match for the well-established relationship of the local sheriff or town guard. The population will likely turn to the local cops to have the party calm down from their adventuring fervor or even look to have them expelled from the area altogether. After all, it’s the weak populace that will pay the price if the PCs stir up the humanoids nearby, when the usual frontier detente was doing just fine. Don’t hesitate to have the local law pay PCs a visit. Maybe a bribe is in order, or assurances that their business won’t take long. In any event, the cops will be looking to keep things quiet, and fighting in the streets is no way to keep the peace.
Local clergy, the ranger in the woods, the wizened alchemist in his tower of bone. All of these folks who dwell in and nearby the PCs current temporary base of operations have a vested interest in keeping the area free of excitement, and may even have greater visions or differing motivations for having the demon lair so close and still active. Modern law enforcement makes a point to keep drug houses under surveillance rather than raid them, knowing that being able to keeps tabs on the criminal element is preferable to driving it underground and losing its trail. Maybe the ranger is steering the raiding giants to greener pastures deeper in the mountains, content to avoid direct conflict with the hopes that his efforts will do what a frontal assault can accomplish without all the bloodshed. And here come’s a party full of combat wizards….
Do the PCs think they are the only ones who happened to read those posting on the board in the major city? There a dozens of groups out there looking to snag that reward of lands, reknown, and booty. Some are certainly unscrupulous and worthy of a shellacking, but what about the parties who do it for the right reasons? What about the good aligned party that is one step closer to chaos, law, or neutrality than the PCs who think that the Demon’s staff needs to be kept for the larger battle? Or destroyed? Or put in a museum? Or cured of its curse and given back the High Right Father Padre? What could start out as an alliance toward a common end could turn into an Isildur-Elrond showdown at the lip of a volcano, with everyone thinking they are doing the right thing.
The party is not the biggest, not the baddest, and certainly not the only good-aligned force with an agenda. Using the people around them, the DungeonMaster stands a chance in keeping their egos in check and their eyes on the prize.