For the love of order
House Tharashk gives you directions to a location about two days (by foot) northwest of Atur. The swamp is fed by Lake Dark and is rumored to be an area rich in magic. Recently, one of the members in Lakeside saw someone who matches the description of Felda Gorlinson (the sister you are trying to locate). He followed her to the edge of the swamp, but was then unable to follow her discretely once she entered the swamp.
The swamp is known to be populated by boggards, stirges, leaches and various snakes. In addition, there are carnivorous plants that are capable of eating a human whole. These are extremely dangerous and should be avoided at all costs. Maral (and anyone with Survival) has an idea about how to identify parts of a swamp that are likely to contain them. Boggards are frogmen, known to make fearful croaking noises. Stirges, leaches and snakes are all capable of inflicting ability damage. You can make a knowledge(local) check to see if you character knows more about boggards or knowledge(nature) to see if they know more about snakes, stirges or leaches. (Specify which when you give me the result of the roll.)
Within the swamp, there is a regular schedule to the weather. For an hour after sunrise, mists rise from the water. At distances beyond 20 feet, everything has concealment (low-light vision and darkvision do not help with this). Beyond 40 feet, everything has total concealment. At two o’clock (give or take a few minutes), it rains for an hour. (You may want to update the daily schedule to accommodate this.) While raining, visibility is half of what it would otherwise be and sound-based perception checks take a -2 penalty. In the best of conditions, visibility is only 160 feet and can be much less.
The terrain itself provides challenges. Much of it will have undercover, providing concealment but also costing double movement to enter. Heavy undercover has a quadruple movement cost but also provides a 30% miss chance (instead of the normal 20%). Undergrowth also hinders stealth and acrobatics. A shallow bog (standing water) also costs double movement and hinders acrobatics and stealth. Deep bogs provide cover (or improved cover if you are small), but require quadruple movement costs (if medium) or swimming (if small).
While uncommon, there is also quicksand. Survival is the skill for detecting it and Swim is the skill for surviving once you are in it.
The full rules for swampy terrain is here.
There are occasional low hills within the swamp, which, while still thick with trees, are not as wet. Any of these might be a good place for a fugitive to try to build a shelter. It is also possible that Felda sought shelter among the boggards. That is unlikely to end well, as boggards are strongly xenophobic and not known for kindness even to other boggards.
At the far end of the swamp, there is an ancient fort. It was built long ago, when the swamp was nothing more than rather soggy grass fields. It sits on the edge of Lake Dark but has been abandoned for several decades. Still, it might be the most comfortable place for someone hiding in the swamp to live.
At Lakeside, they give you a rough map of the swamp. (Good maps do not exist.) The location of known hills and the fort is marked on the map. The green is the swamp, the blue is the lake. (Travel on the lake is dangerous.) The party can travel at 1 hex/hour at normal rates (including the effects of being slowed by the swamp) if they are just passing through and only looking to see what happens to be on their route. Exploring a hex requires 3 hours, after which you can be reasonably confident that there is nothing sizable that you have not seen.