On Shadow, the dark echo of our own world manifests in the Pit. It only takes a little bit of dark magic, or bad luck, for anyone to all into the Shadows. It presses on the dimensional weakness of the Pit at all times. The Navigator had no choice but to include Shadow in the Days of the Week. Of all the interplanar agreements, the Treaty of Passage was the hardest to negotiate. The Dark Lords abide by their word not to invade the Concordance, for now.
Shadow's the worst day of the week. No matter what the Treaty says, somethings always trying to sneak in through the Pit Wall's wards or inside some poor merchant or fool "pilgrim." You gotta make sure you have your silver and cold iron weapons and that you've eaten your adobo the night before!
Day three ushers in a time of caution. The denizens of the Shadowfell arrive. Underneath the Treaty of Passage, they cannot cross the border into the world, but the beings of shadow are not known for following mortal rules. The wards on the wall suffice to keep all but the most powerful at bay.
Before the Orrery, the Shadowfell and Faerie were the most common planes to manifest in the Pit. These dark and light shadows of the world of Erathia press in from either side of the world of mortals.
Shadow sneaks in with the fog. The darkness of night shifts slightly. An obscured sun rises. A deep horn blares. The bass of it pulsing up from the ground through your feet. The defenders on the wall carry silvered weapons. They wear necklaces of garlic and wolvesbane.
The gates open. The shadow merchants ready to trade. The predators ready to feast. Only the brave, the foolhardy or the hopeless brave the darkness.
The Treaty of Passage
After centuries of war between Lozar and the Shadowfell the last Tridenser King made a peace treaty with the Raven Queen and other Lords of Shadow. The creatures of Shadow are forbidden from crossing into the world. In exchange, any who wish it cannot be prevented from walking into the Shadowfell.