The Relentless Herds and Packs

Summary

Upon Veltrona there are many threats and struggles: the ever prevalent Forsaken Undead, extreme weather, disease and sickness, and so forth. However, nothing encapsulates nature's brutality and the horrific unknown together in one complete package as the Relentless.
From any corner of the world they may emerge, often with little warning. Beasts of all kinds, be they simple grass grazers or cunning predators, are consumed by something terrible. No one knows where it begins exactly or how, only that it manifests through convulsions, frothing maws, and overwhelming madness. As it takes root, their bodies bulge, contort, and mutate before a person's very eyes, growing new muscle, bone, and a grotesque figure that embodies one singular purpose: to kill any form of sapient life they can find.
Divorced of anything they might've been, they no longer feel pain, nor a shred of desire to survive. Even eating is an incidental thing; something they spend only a few moments for. They're completely, and utterly, driven by the hunt for sapient life. Though mere beasts they may have been, their newfound strength is simple, yet gruesomely effective. For, even if one is slain, what about ten more behind it? What about a hundred? Their numbers are often vast beyond imagination, with great stampedes and swarms emerging from the wilds to bring ruin to all other life.

Our Eternal Enemy

Found in the oldest of written histories and possibly even beyond that, the Relentless have stalked the world throughout time. Legends and tall tales too numerous to count surround them, all consistent in life-and-death's constant struggle. No one knows where the Relentless truly originate from, or why they even appear as they do. Any wild animal, be it a simple, docile bunny, or a stalking wolf, can become one of them.
It's supremely important to kill any newly found Relentless. If they survive, they somehow induce changes into the surrounding wildlife. Other animals begin mutating, rearranging into yet another form of Relentless. Thus, one becomes two, two becomes four, and the Relentless swell in size with alarming quickness. The sudden, overwhelming surge of these creatures is often what spells the end of people, just as much as their brutality in greater number.
Notably, the available amount of wildlife nearby has a direct correlation to Relentless formation. Areas with less wildlife have much less Relentless, while those with more end up with more. This means the different habitable areas of Veltrona have very different problems with them regularly. Etzli Cuauhtla and Jerhegn, for example, are almost always constantly teeming with one form or another of Relentless. Eylia and Varnkof, having much less to spare, usually only see significant amounts in a span of years, if not decades.
Regional natures being what they are, there is nothing stopping mass Relentless migrations, either. Herds that form inside Immensio's vast plains can easily strike out into Lophern, Temu, Nemma, or Sa-kemet. Already bolstered by their numbers, these sudden and massive attacks are why early warning watchers are something a lot of civilizations employ regularly.
Some scholars have speculated the Relentless were a form of magical mutation, but they're too consistent and too regular to be an 'accident'. A different cause is at the heart of it all, one endless theories try to understand. No end of people, their goddesses as well, have gone in search of the truth. If any of them found it, they never lived to reveal it.
In the end, perhaps that does not matter, for the Relentless must be confronted all the same.
What can be said is that the Relentless follow two distinct themes: that of prey Herds, and predator Packs. In so much the same example, a bunny would be apart of the Herd, while a wolf would be apart of the Pack. Understanding this is vitally important, as their methods differ greatly between the two.

The Stampeding Herds

Of the Herds, they capitalize on numbers, emphasizing brute force tactics. Combined with their extreme physical strengths, the Herds are often a 'hammer' that breaks through walls, gates, and other defenses. Countless cities and villages are trampled underneath, and unlucky survivors would be were torn apart like a child dissecting a bug.
The Herds are distinctly the easiest to notice coming, as they make no effort to hide their great stampedes. They gladly make use of their different sizes for different purposes: large cattle act like battering rams, while smaller goats are head hunters that climb after people more directly. A swarm of rabid bunnies is a terrifying sight unto itself, as they'll dismember a person in seconds. If the creatures they've become are even 'bunnies' in the first place.

The Stalking Packs

Of the Packs, they utilize the cruel cunning of predators to the utmost extreme. Some have even wondered if they, themselves, are not thinking capable given how insidiously clever and capable they are. Packs are often quiet, secretive, or otherwise elusive, silently claiming territory and expanding their number. When they attack, however, not even cities can feel comfortable as these cunning predators slip through every nook and cranny.
For all the advantages they have, impatience is the greatest weakness of the Packs. Their traps and machinations are neither long lasting nor deeply sophisticated. Their successes are more from exploiting moment-to-moment chances, and finding weaknesses people didn't know were there. Many engineers, particularly sokral, consider them the ultimate test in one's defenses as a result. No one has the insidious, self-destructive cunning to tear through fortifications like Pack Relentless do.

Generally, herbivores fall under the Herds, while carnivores are under the Packs. Omnivores, manavores, and other unique forms of consumption more incidentally fall under one or the other. In their cases, its more which side gets ahold of them first that seems to affect the most. This means each Herd or Pack can have very different compositions and quirks, and so pose danger in different ways. It is one of the complicating factors that makes defending against them so challenging to do.
Thankfully, the Herds and Packs rarely cooperate when they run into each other. These two forces meeting often sends them into a self-destructive conflict. In spite of their bestial natures, the destruction they wreak is almost entirely fueled by hatred, not any concept of food or territorial protection. One might mistaken them for the oldest of fueding enemies, if they saw the two sides tearing into each other.
Still, rarely is not 'never', and there are times when truly great civilizations must deal with both forces bearing down upon them. These are often the greatest battles, calamitous events that prove the worthiness of people to survive or fade into nothing. Thus, it can be of no surprise that many ruins dot the world, the hallmarks of people who could not prove superior. Many different names tend to describe these events, but the moniker of the doomtide is the most common one.

The Great Hunt

The Relentless' unending persistence and constant threat means quite a lot of huntresses across Veltrona have permanent employment. Aside from the usual game and wildlife they deal with, they're each charged with hunting down newly forming Relentless when they're found. These brave souls are often the first line of defense, and warning if they're too late to stop a Herd or Pack from forming. Once the call is put out, everyone clenches their proverbial pants, and gets ready for a dreadful confrontation.
It is then that greater organizations come to play, groups of Relentless and professional game huntresses who take on everything Veltrona can throw out. They tackle the worst of the worst, and in exchange have first dibs on the butchery of the creatures they slay. Although the Relentless are unquestionably dangerous, they're a bounty of materials waiting for harvest. Mundane and rare creatures alike comprise their numbers, and their mutations sometimes create highly prized pieces a lot of people would pay for.
Pure destruction isn't necessarily their goal, as it'd waste their pay day in doing so. Whether seen as insane or respected, they tackle the Herds and Packs head on, all with an eye on profit. Only in the worst times, when even these veterans fail to stop the Relentless, that many armies start to muster and prepare their defenses. They hold nothing back, committed entirely to the annihilation of their enemy, or be annihilated in turn.
At such a scale, most start calling the event a doomtide, though some people are too easy with throwing that name around. When even the army begins to be concerned, however, it's a more proper time to invoke that name.

Pets and Farming

As the Relentless can form anywhere and, seemingly, from the vast majority of wildlife, what does that mean for pets? What of domestic livestock upon the many farms? It is a tricky problem, especially as those close-at-heart with their sapient keepers has a direct impact on it.
The mechanism is not understood, but the results very well are. Pets and livestock tended well and cared for can actively resist the Relentless transformation from taking root. Those with a deep and particular bond, such as a childhood companion, even show immunity to the effect in its entirety. Conversely, abused or dispassionate pets and livestock simply transform 'normally'. Some believe they may even have an inclination to actively transform, but it's impossible to form a correlation beyond sheer random chance.
Hence, the keeping and ownership of animals is entailed with many heavy responsibilities. If one will not forge a bond that deep, then they must have the ability and willingness to put down their charges. Otherwise, not only will their livelihoods (and quite possibly lives) be in danger, everyone around them would be as well.
A lot of cultures have clued in on this process, and formed their own answers toward handling it. One particular note is, if an otherwise well-kept family pet transforms one day, it is a sign of abuse or something more sinister going on in the family. Of course, it just as much can be simply random happenstance because a strong enough bond wasn't made. It doesn't stop suspicions from forming, nor doubtful eyes being cast upon the family.
Some forms of livestock farming have even tried deliberately engendering the transformation, as it means access to Relentless materials in doing so. Whether or not its acceptable, disparaged, forbidden, or otherwise, it is not a generally common practice nor one without a lot of problems. Often, one's moral integrity is called into question, if their cultures do not consider it even more taboo.