Goals + Location + Obstacles = Adventure
To begin with the Goals have been very simple - the hint of the unknown and/or treasure!
The locations and obstacles to date have been my megadungeon (which has taken much of my time away from the Pit of Tortured Souls, sorry) and four pre-placed, published adventures.
As the campaign continues, I will begin to use the players to generate the Goals portion of the rumours equation. For example, the magic-user reaches 2nd level and is looking for someone to teach him Read Magic. He approaches the Striped Mage of Hareth but learns that for the Mage to teach it to him, the magic-user must bring back the feet of a Bugbear. The goal then becomes Find Bugbears.
The Location portion of the rumour equation will be handled in one of three likely ways: 1) I will have already placed some Bugbears somewhere in the Northern Marches, 2) I will randomly generate the location for some Bugbears, or 3) I will just pick some place cool to stick some Bugbears.
The Obstacles will also be handled with a similar method. For this example, the obvious one is the Bugbears themselves. But what if they are looking for a Potion of Fire Resistance? I can use the Wandering Monster tables or just pick something I want to use - maybe a Cave Bear.
Things won't be so random for the General Rumours about the megadungeon. As it is the so-called "tent pole" of my campaign it will be much more structured and thought out. And since I want to stress not only the megadungeon but also the exploration of the wilderness of the Northern Marches most of the locations of these General Rumours will be rather small - for example, a 20 room dungeon, a small shrine or a beast's lair.
The final component of these General Rumours will be whether they are true or not. I will use a dice roll to determine if the Goal, Location and/or Obstacles are true or false. So if the party was looking for the legendary sword of Cirso and they heard that it might be found in the Forest of Terror and it is rumoured to be guarded by the Fire-Breathing Giant of Belal, some or all of that may be true. Maybe the sword is in the forest but it is guarded by a normal Fire Giant, maybe the Fire-Breathing Giant of Belal makes his lair in the Forest of Terror but the sword is not there or maybe the Fire-Breathing Giant does guard the sword but his lair is actually in the Mountains of Nyght.
The second type of rumour, the Key Rumours, are specific rumours I have developed about the key figures in the Northern Marches. These rumours can only be obtained from other key figures such as the Lords, Mayors, High Priests, etc. Each key figure has been given two things; a Key Rumour about another key figure and a desire/quest (think video games). By gaining an audience with these key figures and rolling a very positive Reaction Roll, the party can learn either one of these. These Key Rumours are always true and they are designed to tell the story of the Northern Marches. The desires/quests are designed for more significant adventures and may give the party other benefits, for example Duke Belamor desires a famed diamond called the Heart of Zeus. If the party brings it to him he will reward them by making the Fighter with the highest charisma a General of his army. I am also toying with the idea that to learn a Key Rumour from a figure the party must first fulfill the figure's desire (once again think video games).
The Key Rumours may be such things as "the Lady Estir is actually a succubus and is harvesting souls to activate the Gate of Xull and allow her master to enter the world." If you string a bunch of these Key Rumours together you get a picture of the history and goings on in the Northern Marches.
In the last session of the campaign, some of the adventurers travelled to Silverton and heard the rumour that there was an abandoned mine near by that might be the hideout for some bandits (a General Rumour) and had an audience with the Mayor and rolled a very good reaction roll to hear his desire - to find the bandits that have been raiding near by silver mines. If they had rolled a bit better or maybe if they succeed in ridding the area of the bandits, the Mayor may tell them a Key Rumour.