I like Mahjal currently for the name of where the Fire Primordials reside. Something in me wants to pronounce it MAH-zhal.
In describing Mahjal or The Primordial Plane of Fire, it's important to flesh out some of the details of the place. For one, it is not a place composed primarily of fire. One of the races living there is composed of fire, not everything else. In fact, it should be different from the biblical Gehenna or even the Greek idea of Hades.
In the last post, we established that the Fire Primordials eat a native plant rich in sodium chlorate to sustain themselves. They would not necessarily need a lot of water, but could make use of salt water to make "tea". Maybe the native plant grows on great salt lakes like lillypads on ponds. Great rafts of these plants could form in the oceans, drifting around the seas. (Imagine the party having to quest to the Plodding Island of Farouz.)
It was also established that there was less atmospheric oxygen in Mahjal than here. It doesn't necessarily mean *no* oxygen. I imagine it has an atmosphere with the oxygen content found at around 10,000 feet above sea level. I also imagine it might have a higher CO2 concentration than here as well. Maybe we can add varying amounts of other gases like neon and helium. Why neon and helium? It will come up later.
With all the light and heat generated by some of the inhabitants, I imagine that the place is darker, like twilight most of the time. With that, plants won't get most of their food from photosynthesis, so lush fields of grass may exist, but they won't be green. Let's say that the plant colors are darker, maybe dark green, purple or a navy blue. With lower light conditions, other animals will have larger eyes, making a spooky place feel even more spooky. With the lack of light in some places, I imagine a few creatures will take advantage of the neon in the air to produce their own light (but not heat).
To account for the proliferation of Fire Primordials, I imagine that great salt glaciers came down from the colder regions, creating a vast network of salt springs and lakes. Every lake and spring isn't salty, just more than here.
The presence of salt glaciers implies very cold areas, fairly probable for a place with a dim sun (pardon the pun) and/or thicker atmosphere. Just to be different, planets on this plane may rotate on an extreme angle like Uranus. Maybe the great cold areas are found East and West instead of North and South.
All this is great, but what else could be there?
Sodium is a very important element on this plane. Whereas the control of water can create and destroy empires in our history, the control of salt or sodium should do the same here. After all, a sodium compound keeps one of the sentient species alive in the form of sodium chlorate. With so much talk of salt, I would imagine that use of salt in magic would be common. There's nahcolite that could be used as a semi-precious gem or a component of Mahjal's own psionic disciplines.
Still, there's no other species for the Fire Primordials to contend with. Let's put an intelligent people as a rival called the Karnakel. They live in the plains of vegetation mentioned earlier. They live in communities similar to the smaller medieval villages. Karnakel houses are structures similar to the traditional Navajo hogan and some of the newer versions built after the railroads came. Houses are hexagonal or octagonal and made from the ground, stone, nahcolite and other materials.
To be rivals, let's make the Karnakel less aggressive and more artisan than the Fire Primordials. For example, they have more agriculture, stone-working, magic expertise and domesticated animals. Speaking of domesticated animals, the animals used for transportation and building need to be stronger than a horse and possibly shorter. Just to be weird, they could have tails like a platypus and rounder heads like a hippopotamus.
Physically, the Karnakel would have big eyes due to the low light. They would also be a bit shorter and broader. Working with stone implies a greater physical strength. Part of their broadness comes from the large lungs they require to breathe. Unlike the Fire Primordials, the Karnakel do not eat sodium chlorate to stay alive - they exist in a similar fashion to humans on our world. I imagine that they have cultivated plants high in sugar to consume - one of the side benefits being that these sweet plants are a poison to the Fire Primordials.
Now let's bring in the Teleportation Crystals. The crystals stand about seven feet tall and four feet wide. That act as one way gates from one location to another. Walking through one gate will teleport you to the next gate.
Little pieces of the gate act as keys. Pressing the keys to your forehead will teleport you to that gate.
As an aside, I could imagine a Karnakel being directionally confused on the Prime Material plane. Getting used to a sun that rises in the east instead of the north would be a big deal. Then again, the converse would be true of a party of human adventurers with a map of the Plodding Island of Farouz...