Southrons/Haradrim

The residents of Harad are known as either the Haradrim or Southrons. In another case of Tolkien’s wonderful racial ideas, they are described in the Return of the King as “Black men like half trolls with white eyes and red tongues”. In general, they are dark skinned with black hair.
But through comments by Aragorn and Sam in the books and Faramir in the movies, it is suggested they are not wholly evil. It is suggested that they might have been goaded or blackmailed into fighting for Mordor. This is somewhat confirmed by the different reactions to the various evil defeats of Mordor in the War of the Ring. Some fled as one would expect for a group forced to fight. On the other hand too many continued to fight on with too much courage for them to be irredeemably cowardly.
Apparently they mobilized almost their entire populace to fight at Minas Tirith. Their main motives for allying with Mordor was a desire for Gondor’s farmland and vengeance for their treatment by Numenor before it was destroyed. Meanwhile, Mordor’s army gained its best cavalry in the oilphaunts. Not a bad trade. But this did mean the alliance was a risky gamble – loss at Minas Tirith wiped out their entire society.
Which makes sense to a degree. Numenor did conduct raids that captured Southrons for human sacrifices and slaves. But the Black Numenoreans that pushed for the raids that turned the Haradrim into slaves and sacrifices became their allies and masters. Sauron - who was the one who pushed the King's Men into these actions - was the leader of the coalition the Southrons were a part of. Gondor was formed by "the Faithful" Numenoreans who did not fall under Sauron's influence and thus did not push for these actions. So like all good propaganda, there is an element of truth that was twisted into a useful but false narrative. But part of this can also be attributable to the fact that their leadership eventually came from the disgraced former leaders of Numenor who probably justified their defeat by blaming the Faithful for their own idiocy.
So, we have a warlike and proud society. One that exists in a hostile environment. Whose leadership comes from the most militant and pro-Sauron elements of Numenor. Their religion and economics requires taking land and captives. So it seems highly unlikely that they would take a diplomatic approach. It is far more likely that they would use brinkmanship.
Now, it seems extremely unlikely that the Southrons would like the status quo. I would imagine that the Southron participation in the slave trade would not be considered ideal by anyone but the elites in the tribes participating in it. Even they would probably see benefits from trying to get out. After all, the power they need to conduct it and the profits from it require the support of the Corsairs. They also need that support to survive the very real and understandable backlash against them. Gondor's defeat would allow them to settle those lands and transition to farming. Which would allow them to escape the trade. Thus economic security would provide the foundation for physical security. Plus there is the fact that Gondor is a bastion of the status quo, has serious territorial designs on Harad and antipathy to the people. Essentially, the status quo does not benefit them. At all.
So, how would they seek to accomplish their ends? Well, economic resources would be part of it. The Corsairs and Mordor both use slaves. So the Near Harad tribes that I suspect to be the core of the pro-Sauron coalition could use the supply of slaves to their advantage. Other than that, the means of coercion are their only real means. They can conduct raids that would be quite brutal targeting the plantations of the Corsairs for example. But they do not have much that would create a habit of obedience. For example, they worship Sauron, not the other way around.
So, there are three major powers in the area they live in. Umbar, Mordor and Gondor. Umbar is allied to Mordor, is hostile to Gondor and the target of Gondor's expansionist agenda. Mordor is a major land power in the area that could overwhelm Harad easily if it wanted. The alliance between the two provides the powerful basis for a coalition the Southrons can join to defeat their common enemy in the form of Gondor. Or they can form the basis of a pincer move that penetrates deep into Harad. So there is a logic in taking the bandwagon option with the Umbar and Mordor forces.
Yes, this would probably be an uneasy alliance prone to falling apart after the victory over Gondor. After all, the Southern fiefs would be ideal lands for them to colonize. Conflicts arising from this between the Corsairs, Easterlings and Mordor over the distribution of the spoils of war would provide the foundations for future conflicts. Also, the war with Gondor is the glue I see holding them together. After it falling, there is nothing providing a distraction from their other fundamental issues. Like the slave trade.
But two things are of note. The odds of victory over at least one enemy would be much more likely in this coalition. Umbar appears to be too well fortified to fall even with significant investment of resources by the Haradrim. Serious damage can be done to the outlaying plantations, but absolute victory seems unlikely. Mordor is certainly too strong with its orc army several orders of magnitude larger than the whole of Southron society. But due to the common enemy in Gondor and the cultural heritage of the Black Numenoreans, there is the solid foundation for a coalition.
The second thing is that there is no ground for cooperation with Gondor. The two sides will see the other as inherently hostile to the other. Which is understandable. The slave trade, the historic rivalries between the King's Men and the Faithful and centuries of war would make working together difficult. War aims would appear to be contradictory on fundamental issues. Which means that the bandwagon approach to Mordor would be seen as a balancing act against Mordor.

Culture

Culture and cultural heritage

So, their inspiration in the books was unspecified ethnic groups from sub-Saharan Africa. The look in the movies drew more from the Aztecs, plus Middle Eastern or Western Asian people.
The parts of the oilphaunt made them just as valuable dead, with skin, bone and meat providing tools, shelter and food for months. Their dung was also used as building material. Thus, these animals are valuable resources to posses, which led to tribal wars among the Southrons. Part of the reason for the mobility of Haradrim tribes is the huge demand placed on an oilphaunt in the arid climates that make up their homeland.

Shared customary codes and values

They tend to be proud and warlike.

Average technological level

They do not appear to have much in the way of advanced technology. Though it seems likely that this is due to the poor resources of the area preventing economic and thus scientific advancement. Though under Sauron's guidance, they were able to become decent with iron and build stone based buildings.

Common Dress code

The Aztecs had distinctive uniforms for warriors based on their success on the battlefield - measured in captives. After their first, they were allowed a plain shield and a wood club. They could also wear a cape and and specially colored loin cloth. Sandals, a feather suit and a cone shaped hat were worn by those with two captures. Jaguar skins, jewelry, tattoos, body paint and a distinctive hair style were worn by those who captured four enemies. Those with 6 captives to their name shaved their heads and wore face paint. Zulu warriors that killed an enemy in combat got to wear a distinctive head dress.
Hassig, Ross. "Tizoc." Aztec Warfare: Imperial Expansion and Political Control. Norman: University of Oklahoma, 1988. 198. Print.

Clothing appears to be mainly scarlet tunics with gold collars in Southron culture. So I suspect that the traditional Aztec and Zulu patterns would be changed to incorporate these colors.

Art & Architecture

Scarlet cloth and gold appear to be common. Especially on their oilphaunts. Which suggests that this combination is easily available and possibly used as a status symbol. Similar to how purple was used to signal high status in Rome. Red was common on their spear tips and body paints too. Their main standard has a serpent on it, so we can assume that this is an important element in their art.
The Zulu did have settlements called Kraals. Technically, these were more enclosures used to keep cattle inside. Typically, these were within villages. But they were used as barracks when troops were mobilized. The defenses were fences made with thorny branches, mud walls or palisades. There were normally two layers. The inner one just held the cattle. In between the two was the human living areas. Often, their placement was on the side of a hill, with their entrances on the lowest side. The areas closer to the entrances were typically reserved for the more important people in the village and ritual activities.

Common Customs, traditions and rituals

Now, I have no evidence that the Haradrim engaged in human sacrifice. But the "King's Men" that became the dominant class in Southron society was the segment of Numenorean society that did engage in human sacrifice. The Southrons were loyal to Sauron, the same being that pushed the Kings Men into human sacrifice in the first place. Add in the fact that they are confirmed raiders, it makes sense. Not all prisoners could be sold into slavery or fed. I would imagine that the remaining ones would be sacrificed.

Birth & Baptismal Rites

War was a huge part of Aztec life. So when males were born, a shield was placed in their left hand and an arrow in the right. These were then buried hear a great warrior. Each was unique to the family and symbolized the gods. This would be a solid ritual for the Southrons to follow too.

Coming of Age Rites

Aztecs did not consider a person a man until they captured their first enemy soldier.

Funerary and Memorial customs

Aztec warriors who died in battle were burned on the battlefield. An arrow was returned to their home, dressed as the sun god and buried. Mourners would not bathe or shave for 80 days. The wives carried the cloak of the warrior during this period. A wooden box filled with the jewelry would be given to their children.

Ideals

Beauty Ideals

They typically have brown skin. Their black hair is braided with gold.

Related Organizations

• Corsairs of Umbar