A nearly-forgotten world inside Imperium-controlled space, Cuatl III had long been left to its own devices, untouched by the Imperium and xenos alike for centuries. Superficial scans of its surface always revealed what appeared to be a peaceful, idyllic even, biosphere composed in its majority of vast jungles and rivers. The people who called that planet home, humans who had regressed into pre-feudal levels of civilization, knew better.
Living for the most part in upper levels of the extensive, well-made net of underground tunnels left behind by their forefathers and ancient - human or otherwise - civilizations, the world's inhabitants sought refuge from the deadly fauna and flora above them, though even then their population never exceeded the mark of millions of souls. The locals' skin varied in tone according to their exposure to the to the light provided by the twin suns their planet orbited around due to the obstacle posed to sunlight by tightly-packed vegetation; tanned bodies were common even for those who didn't leave their underground shelter often, due to the masterful use of natural lighting found in many "residential" tunnels and the fact few adventured or lived too deep down the earth.
Although Cuatl III's people had mastered growing and harvesting crops which survived in their preferred living areas, much of their sustenance still came from the dangerous jungle. A subject of respect and fear amidst the locals, the wild forest presented a challenge - hunters and gatherers had to know how to run, leap and climb trees, how to survive in a harsh habitat brimming with critters that killed men through poison, claw, maw, trickery or, in some cases, a combination of several natural weapons. The hazards hardened the brave men and women who ventured at the surface world and survived, turning them into survivalists and powerful warriors as the years passed and experience mounted, to the point some could face the planet's natural predators with little more than a stone knife.
However perilous the venom-filled fangs of cold-blooded monsters were to the locals, the most powerful hunter in the jungle had veins filled with warm blood and a thick coat of spotted fur. Elusively agile and flexible, the
large felines that sat comfortably at the top of the food chain were the one of the most terrifying - and possibly the last - sight the humans living there could experience in their lives. With teen specimens easily measuring over nine feet from snout to tail and weighing over 800lbs, and adults easily dwarfing their younger kin to the point some were said to be taller than the largest humans, it wasn't hard to understand why these creatures were both feared and idolized as a symbol of death. Ambush hunters, cunning stalkers in the darkness and nearly impossible to take down in a frontal fight, these animals inspired fighters to draw the strength from their blood, meat and pelt whenever one was killed, to the point all true warriors were expected to have a pelt in their possession.
Cuatl III's culture had with time devolved into something anthropologists would be quick to call barbaric, with the existence of ritualistic sacrifices involving blood and flesh to appease the temper of their primitive gods. The hardened survivors of the jungle's perils were also powerful warriors constantly employed during the occasional inter-tribes disputes and conflicts, which more often than not ended in a number of sacrifices. Armor and weapons were primitive: made of wood, feathers, furs and stone, they were antiquated to the point a "sword" with obsidian teeth was the most impressive piece of the indigenous arsenal.
Though at first glance only a reflex of the degree of ignorance acquired by isolation from the Guiding Light of mankind, the beliefs, traditions, rituals, hierarchy and even architecture of these "savages" were surprisingly complex if studied. Centuries of rock-walled underground tunnels and small temple-like surface buildings told thousands of stories on its carefully carved walls, some of them polished to perfection over decades of careful work by masterful craftsmen. Folk tales, legends and stories gravitated around the powerful warriors who donned the pelts of great slain beasts, their regalia a symbol of strength. Noblemen and priests were reverenced and great works were wrought in their name.
Of course, much changed when priests of the Missionaria Galaxia landed at Cuatl III. Preaching the Emperor's Word with passion, fury, and the assistance of highly advanced weaponry, it didn't take long until more settlements and even small towns were founded by the recently-arrived Imperials as they amassed an ever-growing flock. With faith, technology was also abruptly forced into the lives of those primitive men and women, who soon discovered its marvels. The locals were split between two warring factions: traditionalists (or, as the priests called them, heretics) focused on keeping the old ways untouched and the converted (or "faithful"), who had wholeheartedly embraced the Emperor's Word after introducing some local flavor to its cult.
A bloody and prolonged civil war followed, with vicious under and aboveground carnage mostly performed with the same weapons that had been always used by the locals. Obsidian teeth cut into flesh and clubs were brought down upon bones, filling long, winding tunnels with the dead and the dying. After years of constant, fanatic warfare, the faction faithful to the Imperium prevailed. The planet's population had decreased even further, but Imperial presence had already begun spreading in small, highly fortified pockets in order to avoid the native monstrosities. Small-scale mining and harvesting of rare, valuable materials had already started even as the civil war raged on, and the only difference brought with the end of hostilities was an increase in numbers.
Well over a century after the Ecclesiarch made planetfall, there were still issues which finally had to be addressed after the loyalist victory. Although believers in the Emperor, the converted populace and their descendants still retained most of their culture, including some aspects considered unsavory by the priests, such as their love for certain symbols and strong superstitions. Strong and disposed to labor as they were, the men of the Church still saw their potential as warriors. Judging it a fitting test, a high priest ordered a crusade of sorts: in order to prove their faith to the Emperor and forgive them for the fact their traditionalist kinsmen had lifted arms against the Imperium, the Cuatlans would immediately be trained into the first ranks of the world's PDF and subsequently shipped off to fight in the Emperor's Name.
Time passed and the planet's inhabitants grew accustomed to Imperial presence and technology, though the implements of war were still the only items the population ever took a liking to. Many a tradition of old times still remained alive; the spirit of the jaguar and the flame of combat never truly left the people. The name of the religious figure who ordered the "Cuatlan Crusade", as it became known, has been forgotten but his legacy still lives on due to a mistake that seems to have caused the Ecclesiarch to forget on concluding the nearly abusive manpower tithe. Cuatl III currently possesses outposts much like a Frontier World, greatly focused in exploring whatever they can while avoiding trouble with the local creatures, and up to this day the Cuatlans send a large amount of their young warriors in the hopes that one day their faith will be fully accepted by the church who changed their lives so radically.