Stand at the edge of the Ocean and look out into it. It goes on and on for as far as your eyes can see. It's still but it's moving, it's not human but it's alive. It rolls and tumbles and pushes and pulls, but still retains its shape. The brave sail it, the lazy die in it, and in the darkness are mysteries the human eye has yet to see. Entire ecosystems buried beneath shadows. Shark eyes in a murky cloud of breathless life.
Stand at the edge and look out and know that you're from that place. No matter what you believe, no matter the book you take your meaning from, you know, in that moment, that you are from that place. You were once a lurking mound in the wrinkles of history. You came from there and are now here, looking upon it with wonder, fear, and even excitement. Why? Because you know that you'll return to it someday. As will all things.
Now imagine instead of blue, it is green. That instead of water it is made of leaves, ancient, moss-ridden trunks, timber as old and dusty as time itself. It's a forest, the Wood. Beneath the green canopy is an ecosystem made of instinct; kill or be killed. Apex predatory, prey. On or off. It's very simple and easy to understand. It's that idea we like to call "common sense".
Step inside it and find a world owned by nature. Layer upon layer of human settlements stolen by the forest and turned into ruin. Graveyards of progress. Monuments set upon the hills of misfortune. But still people poke their heads in to see what's going on, and they feel a sense of returning, the guardian gaze of homecoming. It's not a feeling of being watched, it's a feeling of nurture, of destiny. It doesn't feel good, only natural. The Forest knew you would come, and now that you're here, it doesn't want you to leave.
We call it the Fog, because that's what we see when it happens. At some point in your journey in the Wood, you may find something or experience something that causes you to want to leave. You may find the gold needed to retire. You may get the call to return to Aberdeen. Regardless, you will turn around and will to leave the forest. That's when you'll see the Fog.
It will spill into the trees from above and descend upon you. It wants you to come further into the Wood. There is something in there, something so far in and lost, and it wants to be found. It will attack the party in three steps: 1) it seperates, 2) it confuses, 3) it destroys.
Step 1: Locked arms are enough to keep you together, but even inches apart can be stretched to days. Horses detached from their caravans wind up in Druid stews, and comrades are jumbled and scattered like flicks of paper in a hurricane. Anyone seperated from the group, or not attached to one another by some mean, or not touching something, must roll a percentile to determine where the Fog puts them.
Step 2: The Only Road becomes one of many splitting paths made to confuse and muddle the journey. The first time the fog hits, it's four paths, the second time it's six, and the third time it's eight. Roll the dice needed to determine what the right path is, then let the party make their choice. If they choose wrong, roll the percentile to see where they are. The important thing here is, if they are off the Road, they are lost.
Step 3: Creatures that only exist in the Fog come from the Fog to maim and drag those who would leave the Wood, deeper into the Wood. These creatures come in a variety of forms, all of which are familiar to the person they are attacking. Roll to determine: 1) an old friend, 2) a parent, 3) a party member, 4) a sibling, 5) childhood pet, 6) monster from your nightmares. This creature has the same HD as you, the same attack bonus as you, and deals the same damage as you. The second time you meet these creature they have +1 to all of those, third time it's +2.
When does the Fog come for you? It only occurs when you attempt to leave. This is more a state of mind than a location or a destination. Traversing the Forest in any direction is not part of the variable. It is the will to leave that brings the Fog. It comes for you the first day after you begin to return, and through each layer of the Woods as well. Some say it even follows those who make it out.