Overview
A fierce, nomadic race, the Sandriders, as they call themselves, live their lives in the saddle. In fact, the relationship between them and their chosen mounts, the Sandrunners, is so close that it may even be incorrect to refer to them as separate species: each Sandrider is linked to a Sandrunner at birth, and they will remain each other's companion throughout their lives.
Physiology
Sandriders are not particularly different from humans, the major disparity being in the pelvic region, where the bone structure is adapted to sitting on a Sandrunner for long periods of time. They also have more compact and agile bodies, enabling them to swing around on their mounts with ease, and many forego clothing in order to do so even more acrobatically. Sandrunners are similar in size and shape to bears, except slightly larger, sand-coloured and covered with interlocking carapace plates, which can be moved slightly or locked in place. One of the main reasons the Sandriders are nomads is that they can actually sleep inside their Runner's carapace quite comfortably, so they never need to build houses. If speed is absolutely critical, these plates can be shed and will regrow after about six months. Armoured Sandrunners plus their riders can still move very fast compared to more or less any other army on open ground, however.
Both Rider and Runner can go for long periods without food or water, have an incredible ability to sustain themselves in the desert (their staple food being a very exotic but elusive animal that looks a bit like a cross between an alligator and a deer and is capable of photosynthesis), and are very resilient to the effects of the sun; conversely, they hate the cold and large amounts of moisture make them uncomfortable. Reproduction is symmetrical: when it happens, pregnancy will always occur simultaneously in both a female Rider and her Runner, and gestation periods are the same length in both species. The offspring will begin life bonded to one another: twins do occasionally occur, and will result in a four-way link. While Runner and Rider pairs are almost always the same sex, opposite pairings do exist. It is rare that these will end up reproducing, as they will have to find another opposite pairing, only switched around.
Society
The Sandriders are a fractious people by nature, and lack a centralized government. They are divided into tribes which roam the deserts of their homeland, only occasionally meeting with one another. Authority within the tribe ultimately rests with one leader, who is chosen through a ceremony not unlike a jousting match. Feuds between tribes are not uncommon, though few actually die due to overly ritualized combat and a mutual agreement not to use ranged weaponry against one another - they consider it dishonourable to die from an arrow, so not shooting one another is something of a common courtesy. It does not extend to other races, however. While they do not have cities per se, they do have meeting places, and within them, feuds are meaningless.
Despite their lack of urbanization, the desert actually houses great numbers of Sandriders: you just have to know how to find them. When threatened by an outside force, they will invariably gather at their meeting places and set out to meet their enemy once they have gathered enough strength. All matters pertaining to such an event are decided by a hereditary monarch, descendant of a great hero of the Sandriders who united them centuries ago, and in doing so saved them from certain destruction; this is the only real authority the monarch has.
Culture
The Sandriders are, surprisingly, decent philosophers. Chief among their ideas is the power of the spirit, and they believe that the truest expression of the spirit is one's voice. As such, every Sandrider and Sandrunner begins to learn ceremonial chants almost as soon as he can speak. They have chants for more or less every occasion: the Battle Chant is designed such that any singer can stop at any time, so that those who have been wounded so badly that they can no longer sing will not disrupt the chant. Every year a week-long truce is called between all the tribes, during which time the Great Chant is performed everywhere there are enough Sandriders to merit it. It takes several days to perform and incorporates elements of all other chants.
Few outsiders ever hear the Great Chant, but those who do are considered to have known the very soul of the Sandriders. Hundreds of years ago, the date of the Great Chant came while the Great Host gathered by the Sandrider hero, Kunya-Nasab, was still together. That particular Great Chant is a legend that will endure for millennia. In addition to chants, the Sandriders also practice the visual arts, albeit in a limited form: mostly it is limited to war paint, both on a Rider's face and on a Sandrunner's armour. As for crafts, the Sandrunners believe that every skill should be preserved: even though they have no use for carpentry, there may be a few relatively decent carpenters among them: everyone who is skilled in some discipline will attempt to pass it on to someone before he dies.
Magic
No Sandrider has much knowledge of magic, and they do not use any, aside from the telepathic link between every Rider and Runner which defines both of their lives. The moment one dies, the other will immediately commit ritual suicide - for this reason, every Sandrider carries a 'companion sword' at all times, which he will use to cut his own throat should he need to; Sandrunners have a trick where they stand up on their hind legs and snap their own necks. If, for some reason, a Runner or Rider in this situation cannot kill himself, he will degenerate into incoherence, losing all sanity within minutes if not seconds. It is presumed that being severed from one's other half causes immense agony, and prolonging such a situation is as such seen as the worst sin imaginable in Sandrider culture. The source of the spell that sustains the Sandrider link is lost to history, but it is so powerful that it was probably divine in origin.
Combat
Sandriders are great believers in the power of a cavalry charge, and will usually carry several long, thin iron lances into battle. They rarely wear armour, but will carry large shields when their opponents have ranged weapons. Their Sandrunners are ferocious opponents, and tend to break most foot troops in moments. When fighting other mounted foes, Sandriders will often swing on to their opponent's mount: after all, there's little chance of him losing his Sandrunner in the heat of the battle. Sandriders also make good use of mounted archers. When they encounter fortifications, they will always lay siege: they are not comfortable fighting without their Runners, and very few forts are able to facilitate the passage of bear-sized creatures.
History
The Sandriders came to these lands several centuries ago from the East: by that time, all the land they currently inhabit was already desert. Shortly thereafter, an enemy came from the West, across the great mountains, and threatened to destroy the Sandriders. A hero named Kunya-Nasab united the Sandriders and led them to victory. In the aftermath, he achieved apotheosis, and by his peoples' will, ascended to the heavens. He has watched over them until the present day.
The Sandriders are in contact with six races at present: the Cado naso Jiar, the Zefie Confederacy, the Nocturin, the elves in the mountains, the Igni and the Scana.
They consider the Nocturin their allies, and their reliance on the undead is seen not as a shortcoming but as a quality. The use of soulless corpses is considered a sensible, if inglorious way to reduce casualties on the battlefield (after all, the body is merely a shell, according to them), and their power over the spirit, which is seen as sacred and untouchable, inspires awe. Every several years, a delegation of Nocturin priests comes to the Sandrider deserts and takes a few talented youths back with them to be trained as priests; these serve the Nocturin clergy as messengers, ambassadors, field commanders when the Dead Commandant cannot keep his mind on an entire force at once, and the like. The Sandriders are happy to allow their children to be trained for this purpose, as they approve of the training itself and are not nationalistic enough to consider a Sandrider living somewhere else a loss.
The Cado naso Jiar, on the other hand, are considered abhorrent. The Sandriders hate both the cold and the wet, and living inside ice fortresses on the water is the embodiment of both. Further, their unwillingness to face their enemies is seen as both cowardly and dishonourable. The Sandriders refer to the Cado naso Jiar as Icemen, and believe that their evil magic is responsible for turning the moon black.
The occasional Scana trader passes through the desert, and these are given free passage. Once in a while one will ask to hire mercenaries, but this request is almost always rejected.
As for the Zefie Confederacy, while they are aware of its presence to the north, both races have kept more or less entirely to themselves. Hence, neither side knows very much about the other.
Sorry this later than I said it would be, it took forever to write and I've been busy.