Muspellheim
Muspellheim is the Land of Fire. It is one of the first two primal worlds created in a vortex around the World Tree, and the collision between Muspellheim and Niflheim - fire and water, fire and ice, heat and cold - created the energy that formed the basis for the other seven worlds.
Time and Seasons:
There is no day or night in Muspellheim that the average traveler can understand. The Sun and Moon are entirely occluded by smoke, but the light of the fires creates a constant orangey-red sky, rather like Los Angeles at night but more colorful and brighter. The inhabitants actually do have a way of telling time and season, but they don't explain that to outsiders. To the traveler, there may as well be no time or season. Muspellheim spins closest to our world, ironically, at the winter solstice.
Geography:
To say that Muspellheim is the Burning Land is quite literal. A good percentage of it is molten lava rock, and much of the rest is constantly aflame. Attempting to fly over it from most of its borders is nearly impossible, due to the smoke and fumes and long distances where you don't dare land anywhere. The general opinion is that unless you are a fire-etin, Muspellheim is uninhabited and uninhabitable, and no one can journey there anyway.
There is, however, a small portion of the Fiery Realm - perhaps less than a twentieth - where human beings can walk around, with caution. It is the small part of Muspellheim that is coastline; there are beaches of black sand and lava rocks, and bubbling hot springs, and holes in the earth where small fires spring up. Here the fire-etins keep their homes - huts made of shiny black rocks piled and melted together - and they use their human forms here. They are perfectly capable of traveling anywhere else in their world in fiery form, and indeed fire-giants all seem to be more comfortable in fire-form than flesh-form.
The beaches of the Land of Fire are all a long way from any solid border (such as the Myrkwood or the mountainous gates of Svartalfheim), which provides a natural barrier for overflying enemies and spies. Having to cross hundreds of miles of choking smoke, leaping flame, toxic fumes, and devastating heat generally discourages even the most sturdy of flyers. Should anyone make it to the coastal area and actually give in to the temptation to land, they will find that it is well guarded. This is the area of Surt's court, and nothing breathes along that narrow stretch of black beach that he doesn't know about. Generally the fire-giants will be surrounding you and demanding to know your business within minutes, and if they don't like your answer, they will eat you. In other words, don't go there without an appointment with Surt or Sinmora.
The two largest and most impressive structures in the coastal area of Muspellheim - in fact, in all of Muspellheim - are Surt's manor and Naglfari. (Actually, I'm told by the fire-giants that there is a spot somewhere in the uninhabitable zone where they have been working for centuries on a huge spiral design of erupting lava fountains that stretches for a couple of miles, and that it's a really beautiful thing to see, but of course nothing not born in Muspellheim could survive there.) Surt's manor is carved entirely out of a single giant piece of black volcanic glass the size of several city blocks. It is round, with jutting projections like an iron crown, each of them a chimney over a hearth the size of my living room, belching white smoke. There have only been two such structures made, and his manor is the larger one. The smaller one is another giant piece of black volcanic glass, this one with a spiral series of vaulted chambers cut into it, and it is an underground part of Elvidnir, Hel's palace in Helheim, given as tribute by Surt to his godson's daughter.
The stang of Muspellheim - and I know of only one - is the giant blackened half-burnt skull of some mammoth creature, mounted on a pole. It is within visual distance of Surt's manor, in the semi-habitable zone, in the center of a pile of black sand.
Naglfari is the really impressive structure, however. It is a joint effort by Surt and Hel: a giant ship the size of a modern human suspension bridge, built entirely out of the finger- and toenails of the dead people who come to Helheim. It is a little more than half built, and it stands like a frighteningly immense white ghost ship against the stark black sands, directly on the water's edge. The small black sea that laps against Muspellheim moves south over the edge of the world-spiral, and borders only on Helheim. There is talk of messengers who cross the waters, bringing the building materials over cupful by cupful, patiently incorporating them into the enormous ship, glued together with spittle and blood, and then they sail away into the blackness. Naglfari is the Nine-Worlds equivalent of the stockpile of nuclear weapons kept in reserve by two superpowers; each knows what the other has, and that they aren't afraid to use it. Should Ragnarok be necessary - and no one that I spoke to, even the Jotun, seemed to really want things to go that far - they will load Naglfari with all the legions of Helheim and as many Jotunfolk as they can crowd in, and sail on to battle....and the opposing forces will most surely be outnumbered, if perhaps not outgunned.
Flora:
None to speak of. Muspellheim borders on the Myrkwood, which is more properly a liminal space between the worlds, and there is vegetation there, but the Muspellheim side of the Myrkwood is full of charred trees.
Fauna:
There are creatures in Muspellheim, besides the fire-etins, but little is known about them. There have been sightings of reptilian creatures that crawl through lava-hot mud, varying in size from a couple of inches long to something the size of a city bus. The fire-etins do hunt and eat some sort of animal; there are charred bones around to prove it, but discussing their diets is considered to be impolite.
Cautions:
Burns, obviously. Clothing and gear should be marked with spells or bind runes of nonflammability. Bring a salve of healing herbs - houseleek, calendula, and plantain are good - charged with healing energy, but expect to wake up with some amount of blistering anyway. Heatstroke is also a problem - bring a lot of water, perhaps marked with the Isa rune. It is said that the best water to bring to Muspellheim is melted snow from Niflheim.
Residents:
The fire-etins are territorial, and curt and abrupt with outsiders. They take offense fairly easily, and react in an appropriately fiery manner. It is best not to travel there without first getting permission. In their human form, they stand six to eight feet tall, and their skins are usually blackened with soot. When they flip to their fiery forms, the soot is shaken off, and so you can see them with unblackened skin for a little while after they change back. They wear very little clothing - usually just a tunic or loincloth of some sort of tanned reptile skin - and there seems to be no clothing difference between males and females. In their fiery form, they are like great pillars of fire, sometimes vaguely humanoid-shaped, and sometimes not. They can fling fireballs a good way, so running away from them is not recommended, nor is attempting to fly in. Like all etins, they are cannibals, and are not averse to eating visitors. Unlike other etins, they eat all their food cooked...because they can cook it in a matter of seconds.
Fire-etins are generally cheerful and wild, except when they are being suspicious and cautious. In fact, if they are subdued, you're probably in trouble. They have infectious laughter and love to shoot sparks, competing with each other to create fireworks that illustrate their moods. They are the most confident and courageous of etins, and they always laugh during battle. They do some of their own metalworking, but for the more intricate and delicate things they trade with the duergar, who are the undisputed masters of forging.
The Lord of all fire-etins is Surt, and the Lady is Sinmora. Some claim that they are husband and wife; some claim that they are the same individual, and that Surt happens to like flipping from male to female form, and having a separate female persona. It is true that I've never seen them both at the same time, but it's another issue that I don't think it's polite to bring up and discuss. Surt is rather short for a fire-etin, which shows his great age - not that they get shorter with age, but more that the younger generations are taller. His manners are more courtly than the average fire-etin, and his wrath is a little more controlled. He is very intelligent, although he has on occasion acted less so in order to gall visitors into making rude comments, and thus having a reason to fry and eat them. He is not to be underestimated. Surt has a close relationship with (and a great respect for) Hela, with whom he is building Naglfari as a joint project. He has said that he is Loki's godfather, and that Laufey came to Muspellheim to give birth to him, because nowhere else in the Nine Worlds was hot enough, and that she lay in Surt's biggest fireplace to bring him forth.
Fire-etins are useful for learning to work with fire - well, obviously. They can help you with learning to make fire from an older method, such as tinder and flint, and with deciphering the subtleties of the rune Kano/Kaunaz/Ken, and with learning to heat your body with your own energy. They are very good for people who tend to throttle their aggression to the point where they get stepped on; they can help folk get in touch with their inner fire. They are also good for people suffering from burnout, who have lost enthusiasm for life. Their courage and confidence is contagious, almost to the point of blind enthusiasm, but it's a nice change for the tired and cynical.
Offerings:
Food that they wouldn't normally get - raw fruits and vegetables, especially if they are full of juice, like citrus. Don't comment if they cook or char food offerings that we tend to eat raw. Whole grains and ale poured into a fire is another good offering.