The campaign rules on page 31 of the DKQ quest book include a simple balancing mechanism to tailor things to your group.
The balance of the metagame is very sensitive to the relative skills of the different players, especially how cunning the Overlord is compared to the Heroes. Playing the Overlord and the Heroes requires different skills and mindsets, and people are usually better at one side or the other. Sometimes this means that the Heroes romp through with little to stop them; other times the Overlord halts them at every turn. Ideally you have a balance, but this isn’t always the case.
The campaign rules give the Heroes 15 campaign hours (tries) to get through all the adventures. During testing this was the average needed, and gave us about a 50/50 split of victories. However, as I mentioned above, the relative skills of the players often made a big difference, and you may want to tweak this if one side is especially cunning or inexperienced compared to the other.The easiest way to balance a campaign is to change the number of campaign hours available. You can either do this by simply picking what you think will work better for your group, or you can bid.
Bidding works well when you have more than one person who wants to play a particular side. One of the players declares that they can win with a particular number of campaign hours on the clock (you could start with 15). Then one of the other players who would like that role declares that he can do it in less/more (depending on which role they’re bidding for). Continue until all players bar one have dropped out. The last player then gets to try to live up to this boast as you play through the campaign.
Obviously it will depend on which role the players are bidding for as to whether the campaign hours bid should be going up or down. Bidding for the Overlord means offering higher numbers (meaning you have to hold out for longer).
Incidentally, the most common mistake of inexperienced Overlords is to care too much about their minions. Think big – the individual lives of your followers don’t matter. What is important is killing Heroes, and if that is proving too hard, simply slowing them up enough to allow your Great Work to be completed. In a campaign sense, this means slowing them down so much that they can’t get through an adventure before they run out of time. In reality, time running out simply means that reinforcements have arrived from deeper in the catacombs, and the Heroes are forced to retreat by a wall of enemies, and wait for them to return to their lairs once more before they can try again.
One final thought on these campaigns: people occasionally say that the Overlord misses out on the extra mini-missions. That’s right, he does, and quite rightly too. Apart from balance issues, this goes back to the difference in ways of thinking. We’re used to seeing movies and reading books where the Bad Guy is rather less detailed than the Heroes, and D&D et al has taught us that Heroes should be worried about the minutiae of equipment. The Overlord is appropriately rewarded by the campaign timer ticking down every time he wins, and this long view is where he needs his head at. His minions are merely the means to an end and individuals don’t matter, so nor does their exact equipment – the Overlord has far more important things to be pondering. Heroes, on the other hand, are all about the individual and really care who has the shiniest sword :)