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 unimpeded. 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 foods being a variety of tough plants, some of which are left to grow in hidden grottos by the Sandriders and are visited a few times each year, and a very elusive animal called a jarak which looks vaguely like 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, though 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. Blood 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.
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 a result, every Sandrider and Sandrunner begins to learn ceremonial chants almost as soon as they 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 among the Sandriders. 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 body and on a Sandrunner's armour. As for crafts, while their resources are somewhat limited, they do have some skilled artisans. Manufactured goods are at a premium, and feuds are often motivated by disputes over them.
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 within 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 at least one long, thin iron lance into battle. They rarely wear armour, but will carry large hide 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.
The Sandriders did not always have a patron deity. Centuries ago, they were engaged in a war with an enemy whose identity is now lost to history. Defeat seemed certain for the scattered and disunited tribes, until a Sandrider named Kunya-Nasab united them and led them to victory, saving the Sandriders from extinction in the process. By his people's will, he achieved apotheosis in the aftermath, ascending to the heavens to become their guardian.
Now that the other Gods have returned to this world, Kunya-Nasab has decided to return as well in order to protect his people again. He hopes that he will not need to intervene, but he has already chosen a prophet: a young Sandrider girl named Saira. She wanders the desert with her small tribe, and has begun preaching the return of the Gods.