Now that you have started building your world for your D&D campaign, its time to start thinking about the campaign arc -- or plot if you will. The campaign arc is every bit as important (if not moreso) than the world itself, and is a big reason why many DM's will choose to use a published campaign setting for their game, i.e. there's enough work involved in the campaign arc that its often easier to use a published setting. Plus, you could have a really amazing homebrewed world that's full of life and flavor, but if you don't have a plot, your campaign is still likely to be pretty blah. Some groups will be perfectly fine going from one random encounter to the next, but most will want at least some semblance of a plot thrown in.
What's the Lifespan of Your Game?
The first thing you need to do when preparing you campaign arc is determine how long (in real world time) your game is going to run, and how many sessions (roughly) you're expecting to have. By way of example, if you are preparing to run a game for your friends over the Summer before everyone goes back to their respective colleges, and you anticipate meeting once a week for 4 hours at a time, you are looking at roughly 12 four hour sessions. What this means is that there is absolutely no point in drawing up a story arc that will take the characters from level 1 to level 30*. At four hour sessions, you probably will not get enough encounters in to level up even after every session. Even if you did give the characters a level after every session, you're still looking at only 12 levels. By contrast, if your game is going to meet once a week for 8 hours for the foreseeable future, you are pretty wide open in terms of the scope of your arc. It is important to note though that just because you envision being able to meet for 8 hours a week for 3 years doesn't mean you need to plan your arc to take up that entire span.
What's the Problem?
A good campaign has some kind of a problem that the PCs need to solve. Maybe its repelling a Githyanki invasion, or stopping Orcus from usurping the Raven Queen's power and authority. Whatever it is, ultimately there has to be something for the PCs to fix. Adventurers are a rare breed so their problem needs to be something that can't be overcome by the regular citizenry. Building an aqueduct to divert water from the sea to the fields of Greenbrier is not a PC problem. Saving Pelor from the schemes of Asmodeus or Tharziduun so that he can shine light upon the fields is a PC problem. This problem is important as it will define your campaign.
I should note though, that even though this problem is important, the nature of it is not necessarily relevant to the PCs at the beginning. In other words, the problem could be many-fold such that when the PCs start their careers they start out to solve one problem, not realizing that its just one small pawn in the schemes of the Big Bad Evil Guy (BBEG). Take the Scales of War campaign for example. There are essentially three main problems in SOW that are all connected in some way. At the start of the campaign, the PCs are only going to know about one of those problems. If you are going to break your problem up this way though, you have to make sure that the first problem is significant enough to spur the PCs to action.
Once you have your problem, its time to start outlining your campaign arc. James Wyatt goes into pretty good detail about this outline in Dungeoncraft, and it was reprinted into DMG 2. The advice there is pretty solid so I'm just going to summarize here. The long and short of it is that you need a rough idea of what your PCs will be doing over the course of the campaign. Knowing when the next piece of the puzzle will fall into place will help direct you in your adventure design. An outline of "The PCs will start off fighting some undead, then eventually encounter some aberrations, before finally fighting Orcus" won't get you very far in your adventure design. Although this could potentially make for a very cool campaign, you need to know when will the aberrations make an appearance, and when will Orcus' involvement become known. Of course, in this situation, you also will need to explain how Orcus is connected to aberrations since that is not nearly as obvious a connection as the undead. All of this should go into your outline.
Detailed But Flexible
I recommend making this outline fairly flexible. While this is not the time to start writing out all of the adventures of your campaign, you should at least have some idea of what your PCs will be doing at each level. The thing is though, you need to be flexible (which is why you don't want to write the adventures yet). The problem is, no matter how detailed or carefully executed it is, no plan ever survives first contact with the PCs. Being the DM is in many ways like being a producer or director of an improv drama group. You create the set, and you might even give them some suggestions along the way, but ultimately, the action, and the story, is dictated by the PCs.
You've got a great plot that has the PCs exploring the caves of Ironhome and then travelling to Greenbrier? Great, except for when the PCs decide to travel to Hawthorne Ridge instead. As the DM, you really can't tell them "No, you go to Greenbrier." More to the point, the PCs may end up levelling faster or slower than you anticipated, thus causing you to have to modify your outline anyway.
Story Within the Story
As alluded to earlier, often a good campaign will have a series of related problems that together define the campaign. Note that while its likely that most if not all of your adventures will be related, the adventures themselves are not the problems I speak of (though as I'll discuss in a future blog, adventures also start with the same "What's the Problem?" question). Rather the related problems should probably be around 2-4 at most. The idea here is that it gives the PCs a sense of accomplishment during the course of the campaign. If the PCs spend 30 levels chasing the same nemesis, its likely to get frustrating and boring. If; however, they catch up to their early nemesis at say the end of Heroic Tier, only to discover that he was just the tip of the iceberg, it still gives them that sense of closure, while also spurring them on forward. Personally, I think that a rough divide of one main problem for each tier works pretty well. Letting the PCs finally solve the problem as they move into the next tier makes for a pretty good transition. Think of it this way "Yay! We saved the Valley . . . Oh crud, now we need to save the rest of the country."
Listen to Your Players
This is going to be a recurring piece of advice. You need to listen to what interests your players. Maybe they could care less about the tax dispute between the blacksmiths guild and the tanners guild. Even if you have a really great campaign planned around this problem, PCs will never experience it because the players just don't care. While its likely not surprising that a group of players is not interested in a tax dispute, the issue here is that you never can tell for sure what will motivate or interest your players.
Filling the Gaps
Odds are that in your first brush through of the outline, you are going to have some gaps. While its not ideal, its also not fatal. As long as you have a decent idea of where your campaign is going, you can fill in the gaps at a later time, though I do recommend trying to fill those gaps sooner rather than later.
So what did I do? Well obviously for the Scales of War game, this part was already taken care of as the campaign was already outlined and in fact completely published by the time the group started up. However, its worth noting (as I did above) that SOW has three main subplots. Each of them roughly (though not perfectly) lining up with one tier of play. I won't spoil what those plots for those who may be playing SOW, but anyone who wants to can easily learn this info.
For my other game, the basic problem is the plot to recover a powerful primordial artifact from the Dawn War. The PCs learned this information very early on in the campaign as well as learning that another artifact was forged by six gods to help defeat the primordial during the Dawn War. The course of the heroic tier has been spent chasing down the BBEG who is believed to be orchastrating this plot. At the close of the tier, a new enemy will emerge who will be the focus of the paragon tier, with the epic tier being a fight to prevent the ressurrection of the powerful primordial. I do have this laid out in much greater detail but there's no need to fill this space with that info. The point is that each tier will roughly wrap up a story with a nice bow. I feel that this is good in that if the game fails to last throughout the entire arc (a realistic fear given that the campaign is a year old and still in heroic), at least the players will have a chance for some closure.
That's all for today.
*I should note that you could potentially do a 1 - 30 campaign by giving out large level jumps along the way. "Great that wraps up level 6 guys, next week have your characters levelled up to 11".