Happy New Year! Well, in this world, anyway. The world of Mor-Thir has a few months yet.
Anyway, on to the main event. As far as the story of Mareten, Mor-Thir, and beyond is concerned, there are three sets of truth: "truth", truth, and TRUTH. And what, you ask, is the difference? Well, keep reading!
History may never change, but the stories about it certainly do. Some stories are ancient, so old that they have lost something in translation; other stories are passed along from bard to bard, and gain a few details along the way. Still others are pieced together from ancient texts, snippets of poetry, and nigh-on-incomprehensible journals, and are more guesswork than eye-witness accounts.
While there is a note of truth in many old stories, take them with a grain of salt - if you find facts that counter an ancient story, it's likely your facts are correct. Don't dismiss the ancient tales out-of-hand, but... don't assume they are exactly accurate, either.
Facts, when interpreted in a different light, can change a story dramatically. Details get lost, facts are twisted, and eye-witness accounts are affected by faulty memory. Most likely, however, the story you hear is from the perspective of the narrator. If the mayor tells you a witch has been spotted outside of town, and it coincided with the disappearance of several children, it is likely true; however, he may have forgot to mention that those children are always running off, and that the witch has lived peacefully outside of town for decades. Maybe she did kidnap those kids... or maybe the mayor just doesn't like her, and can finally pin her demise on a bunch of blundering adventurers. Truth, in this case, is selective.
Information received from NPCs has a chance to be false; only investigation can prove rumors or statements, and only trust can keep you from abject paranoia. Luckily, ESP and other spells and abilities can narrow down which statements are truth, and which are lies.
The one story that is absolutely, entirely, and in all other ways true is the account of the actions of players here. I don't play any silly "It was all a dream" games; actions that happen, be they to the players in the present, the past, or the future, are entirely factual. If actions taken in the past affect the carefully planned story I've laid out... well, then I'll rewrite my story! One-shot or long-running campaign, the circumstances and choices of the players are set in stone.
Well, unless I introduce time travel.
...Then we'll see.