Paraelemental Plane of Smoke
The paraelemental plane of smoke, located between the elemental planes of Air and Fire, is an astoundingly unpleasant place for any non-natives to travel through, filled with gasses both poisonous and toxic, visible and invisible. Though it is survivable, it is only barely so, and few outsiders choose to travel through its bounds unless absolutely necessary.
Unlike many Inner Planes, most of the plane of Smoke is survivable by mortals, if uncomfortable. Thanks to its proximity to the plane of Air, the majority of this plane is filled with breathable air itself, though air contaminated by sooty, and over time harmful, vapors. In the short term, this poses no harm to travelers; in the long term, for every hour spent upon the plane of smoke by those not immune to poisons or gasses, an individual must make a DC 15 Fortitude save (the DC increasing by 1 for each hour beyond the first) or take 1d6 points of nonlethal damage (which can be healed). Further, every day spent on Smoke by such individuals gives 1 point of Con damage, which cannot be restored by natural means until leaving the plane. Of course, there are even more dangerous regions here and there; scattered about are pockets of even more deadly gas, vapors that can suffocate, poison, or even explode. Any form of harmful vapor or gas can be found somewhere on the plane, from the corrosive airs of acids to the shimmering firedamp found in the deepest mine shafts.
Thankfully, for the most part this plane's proximity to Fire has little effect beyond a generally high temperature. The entire plane usually holds around 80-90 degrees Fahrenheit; not enough to cause harm, but enough to cause discomfort to most. Of course, as with the air, there is the occasional pocket of higher temperature, to the point of danger. Thankfully, unlike the near-invisible pockets, these regions can be well noticed long before their heat reaches a dangerous level. And of course, there is also the ever-present soot, which tends to leave all visitors looking like a chimney-sweep upon their departure.
The nearness of Fire also prevents this plane from being a lightless void, though it does quite well on its own at restricting vision. A combination of the thick haze and the eye-stinging soot prevents sight of much beyond 30 feet; further for natives, of course. Eye protection can help to some degree, but even with its benefit, vision beyond 60 feet is impossible for outsiders.
Despite all this, some do try and make their living here; as mentioned above, all harmful vapors can be found somewhere on the plane, leading some to harvest and gather these vapors for sale elsewhere. Many entrepreneurs, but especially the efreet, "mine" the plane for both fuels and toxins, distributing them across the planes at great profit. Of course, this is far from safe, and thousands of harvesters die each year from either exposure to the plane, or accidents with the devices (both magical and technological) used to collect and separate the product. This hardly dissuades the efreet, exploiting slave work for most harvesting as it is, but neither does it dissuade those seeking to make their fortune on the vapor farms.
Travel on the plane of Smoke is the same as on many Inner Planes, based on manipulating the subjectivity of gravity. As on Air, there are various simpler and safer means of travel also used, and there is in fact a very profitable ferrying business on Smoke as compared to Air, given the ease of extracting fuel and balloon-gas from the plane itself. With many of the largest such pockets charted, many trade lines ply the plane, transporting outsiders in enclosed gondolas with clean, fresh air, and though there is the occasional disaster, the convenience more than makes up for it for most visiting travelers.
The sole solid surfaces to be found on the plane are the occasional cinder islands, which can range from a few feet across to hundreds of yards. Excepting the rare cluster, these tend to be solitary islands, and most are claimed by the locals - belkers especially prefer to live in tunnels within cinders. However, on occasion small mortal settlements or, more often, single buildings are built upon unused or captured cinders, used as way-stations, fortresses, or outposts for mortal guests.
Much as the rest of the secondary Inner Planes, there is little common access to Smoke. Elemental vortices can be found only in areas of heavy smoke or gas, none of which tend to be especially safe - the regions above undersea volcanic vents or heavy forest fires are the most common, but they have been reported in the deadly, suffocating valleys that can be found around recent volcanic eruptions as well. As for well known planar portals, the most common is the Bytopian gate said to have been constructed by Garl Glittergold and Flandal Steelskin. Keyed to gnomes, the only ones allowed to use this gateway are gnomes themselves, or those that prove themselves of good character (as judged by Bytopian standards, of course). Less well-known is the Smoke Ring, a construct of solid iron in the realm of Moradin in Mount Celestia built by the god himself. This gate opens only at the willing touch of Moradin himself, or one of his priests or proxies, and thus getting entry through it can be even more difficult than the Bytopian gate.
While its elemental energies are not concentrated enough to empower either Air or Fire spells, Smoke's elemental nature does cause spells of the Earth or Water descriptors to be impeded.