The Nomads:
The nomads call themselves the "Kadiz" whereas members of the Orthocracy are the "Kaddish". This is a shibboleth between the two groups. They speak mutually intelligible dialects of the same language, Kaddish/Kadiz.
History:
The old kingdom of Kaddish was an oligarchic monarchy, with a king who controlled the city and various nobles and officials who controlled the surrounding countryside. When the revolution happened, about a quarter of the city was driven out of the kingdom (the nobles, the surviving royal family, their servants, and anyone thought generally to support them) and fled onto the plains. There, they allied with friendly clans of Hill Elves whom they had previously supported against more hostile elements.
Because the nobles left much of their wealth behind and because they were blamed for provoking the revolution, their power was shattered, and the Kaddish refugee social system began to fall apart rapidly as the newly-emerging "Kadiz" blended elements of Kaddish and Hill Elf culture.
Races:
The nomads of the Plains of Kadiz are the remnants of the old oligarchic party of Kaddish mixed with the more civilised Hill Elf tribes. They have intermingled for about a hundred and fifty years (since the revolution drove them out). Humans and elves continue to form distinct ethnic subgroups within the nomads, but trade and intermarriage mean that the two races are co-extensive with one another - a clan may be mostly elf, but it will have a few human members, and at least as many half-elves. Humans are the more populous of the three races, with half-elves next and true Hill Elves last. The Hill Elves that joined with the nomads are the old traitor clans who assisted the Kaddish against their fellow elves in exchange for the power that an alliance could bring.
Few other races are found amongst the nomads. The knowledge and tools required to soulforge races have been lost by them. Anyone else is almost certainly a slave, a trader, or a refugee from one of the cities. The nomads only allow refugees to join their society if they can be sure that they aren't spies - helots from Dwer Tor and political exiles from Kaddish stand the best chances.
Religion:
The rootstock religion of the nomads is the old Hill Elf religion. It features two main groups of spirits to be placated - the Storm Bulls and the Wolves of the Earth. The wolves hunt the bulls, which causes phenomena like meteor strikes, prairie flash storms, earthquakes and such. The nomads sacrifice cattle to the Wolves of the Earth to feed them and keep them from chasing the Storm Bulls. They burn great swathes of grass as sacrificial feed for the Storm Bulls. There are two different types of priests, each responsible for appeasing one group of the spirits and performing the necessary sacrifices. Generally, priests are not professionals. Priesthood is awarded as a title to worthy members of the community who know the rituals.
The Kaddish brought with them the Knowing, which is understood as part of the Kadiz religion despite actually being wizardry. A Knower is simply someone who understands how arcane magic works well enough to cast it. One may be both a Knower and a priest, and this is in fact quite common.
Most clerics are actually shamans. A shaman is seen to be under the purview of a spirit that is not a member of the Celestial Herd or the Stone Pack. These kind daimons help or harm mankind for reasons that are clear only to them and perhaps the shaman. Most powerful clans keep a few around, but otherwise shamans are expected to wander the plains as they please.
Travel:
Briefly, humans ride horses, elves walk. Not all humans own horses, but horse-taming is a skill humans brought with them to the elves, and the great herds remain in the hands of majority-human clans. Wagons are common, as are sledges.
Families and Clans:
Kadiz families are large, complicated, and confusing. The Hill Elves consider everyone in a clan to be related. This meant that to marry someone other than a relative, young elves would raid other tribes for wives. The humans brought the polygamous, matriarchal practices of their ancestors with them, and combined them with the Hill Elvish clan structure.
In practice, generally one man (elf or human) is in charge of each sept of the clan. He has many wives of both elvish, human and mixed descent, and attempts to father as many children as possible on them. The male children must go out raiding or trading to find as many wives as possible, while the female children are traded out to other clans or other septs by the older women. The wives of dead men are usually married off within the clan to other men, usually friends or brothers of the chief. Once a man has enough wives, he may start his own sept. Members of the sept and the rest of the root-clan are considered only distantly related to one another.
The men are generally so busy raiding or herding or praying that the women actually run the sept, and most inter-sept business within a clan is brokered by them. All wife trading is handled by women with minimal input from the man allowed.
This is a highly unstable system, and it is only the high mortality rate amongst men, the large families of the nomads, and the fact that they raid not just other clans but also the Kaddish, the helots of Dwer Tor, and hostile Hill Elf clans for any women they can find that keeps it functioning. Like most polygamous societies, there is a fairly stark dichotomy with some men having many wives, many children and much status in society, and many men having no wives at all. These men tend to form the bulk of the raiding parties sent out to capture new wives.
The nomads have very large families. Child mortality is low because the diseases that plagued their urban ancestors are not as easily transmitted with the lower population density. Food, in the form of cattle, sheep, goats and their byproducts are plentiful. The polygamous structure of families also means that four or five children can be carried to term within the same period of time. Having large families is culturally expected, and eventually founding one's own sept is desirable.
The inhabitants of Dwer Tor and the Orthocracy both have stereotypes of the Kadiz as sex-crazed murderous bandits that derive at least partly from these customs.
With the PCs, this is mostly going to mean that they're all either going to be unmarried goons who'll feel constant pressure to get married, thus leading to adventure! And they'll have huge families who are always getting into trouble and need their adventurer third-cousin-twice-removed to help them get out of it.
More later.