As the name implies, the fimbulmist is an apparently infinitely deep bank of white fog or mist that encircles all the lands of Farforth, isolating the known world from the unknown regions beyond. The fimbulmist does not extend infinitely into the sky, however, as the sun and moon and stars are still visible above. Banks of fimbulmist can take on many of the same forms as clouds do. For this reason, sages suggest that the whole of Farforth may actually sit atop some sort of huge plateau that scrapes the natural cloud cover of a far larger, underlying world. Whatever the case, no one has ever returned from beyond the mists to confirm or deny this hypothesis, and not even magic - including the powers of flight, teleportation, scrying, or immunity to illusions - has permitted anyone to pierce the veil and witness the unknown.
The fimbulmist banks are not entirely stationary, as particularly cold or humid weather - especially in the northern and southern extents of the bounded region known as the 'world' - can cause the banks to move inward or even form long, tendril-like incursions into surrounding lands. Travellers will sometimes become disoriented within these tendrils, expressing a phantasmal sense of 'losing oneself'; these travellers will usually be safe if the tendril receeds or they manage to navigate away, though they are more prone to following the tendril back to the actually deleterious boundary fimulmist banks. Conversely, on particularly warm days, the fimbulmist reaches higher into the sky while the regions closer to the ground become less dense, exposing unfamiliar lands to observation for a time. Explorers visiting the extents of the known world during these conditions have reported sightings of strange humanoids moving in the indistinct distance, appearing to watch explorers with interest until the fog's strength returns or the explorers move away. These 'mist walkers' are said to appear as though made of pure white cloth and move with an eerie, boneless quality.