Wild Courts

Wild Courts are the representatives of the Fae, the overlords of majority (if not the entirety) of the Lost Lands. Who however tend to remain more or less irrelevant save - they are the spirits of nature who can technically be encountered in any forest in Karadia, however whose behaviour will greatly differ depending on many factors. In most cases they will be pretty much a part of a greatly expanded magical ecosystem, without any deeper thoughts behind their actions.
This changes in the territories that are truly abundant in nature, who are controlled by the Wild Lords, the most potent of the Fae and the patrons of all the Wild Courts (although it can also be that these places are abundant in nature BECAUSE they are controlled by the Wild Lords, the opinions vary). These territories will vary greatly between the Courts who ruled over them, as every details of the local climate and the ecosystem is entirely up to decisions of their Wild Lord.
A strangely flowery forest filled with friendly animals, where mortal servants of the Courts frolick in eternal gentle summer. A hell of thorns, poisons and physical strength, where only the strongest survive and the place of the mortal servants is that of a part of the ecosystem (one far from the top). Or something exotic, like a jungle or a perpetual tundra (despite being located deep inside of Karadia). Or something way more alien, like a mushroom or crystal forest? The possibilities are endless.
Unfortunately to the world, Wild Courts are often utterly alien in their mindsets. It isn't unheard for them to consider simple animals or animalistic beasts as equal to their mortal servants - and that's only the surface level of the problems, as they are as likely to appear as utterly understanding and full of kindness to all life, as they are to be personifying the selfish 'survival of the fittest' and utter amorality towards all life. They are the Nature, being all of its faces at once. These makes any attempts to do diplomacy with them a rather troublesome endeavour.
It's unknown when and where exactly the Wild Courts started. Many suspect them to be ancient elven deities of nature, while some think that they even older and actually predate the Xylian Dominion. The Fae of the Wild Courts tend to be tight-lipped on that field, and remain so for millenia. The first historically confirmed mention of the Wild Courts is during the Forest Wars, when the ancient elven countries attempted to break the stranglehold of the Wild Courts over significant part of the Karadian landmass (and failed). However the Wild Courts were already deeply entrenched back then, and while they were believed to have finally mostly succumbed to the Second Karadian Empire, they appear to back in force in modern times.

Structure

There is always the Wild Lord himself/herself/something else who is ruling over the segment of the Lost Lands. However their level of power and the fact that their 'segment' is almost always centered on themselves, makes them rarely leave their seat of power. This means that all the lifeforms - from the small insects on the ground through the bushes and trees to everything that moves - is left to be governed by the Wild Courts. Who are composed of various powerful fae, with their exact fields of responsibility typically being completely abstract to mortals.
A True Fae of the Wild Court can for example be 'tasked' with overseeing the reproduction cycle of some type of local insect. The degree of their power and how exactly they perform their supposed job is anyone's guess (it certainly doesn't depend on how subjectively important their 'role' is). Some scholars go as far as to suspect that the True Fae simply treat such things as honourable titles, and are only loafing around the courts, lording over the lesser fae (and other citizens) and sometimes defending their little 'kingdom' from external incursions.
Beneath that level are the 'citizens', be it the lesser fae, beasts, or the mortals who ended up being subsumed by the Wild Courts at some point in the past (primarily the descendants of the elven captives from the Forest Wars and ogres who some suspect sold themselves to the Fae while the Xylian Dominion still existed.

Military

The Fae tend to be fickle and act like they are mentally unstable, but are just as likely to be stable in their believes and desires to an unhealthy extreme (they are, in short, unpredictable). They come in numerous forms - sometime they beautiful maidens who can control the forests around them, sometimes they are what appears to be a walking tree stump, and sometimes they are almost impossible to be discerned from the beasts.
Stronger fae are capable of ordering around animals and lesser beasts, which makes them serve as field commanders of the armies of their Wild Court. However even when you take a single 'type' of Fae into account, they will be widely different in combat depending on the nature of their Court. This includes even the Fae who live outside of the core Wild Court territory, as you can encounter them almost anywhere (and all are members of the most nearby Court, even if rather estranged ones), although they will tend to resemble their own habitat rather than one of the Court.
Beasts are special types of animals who are at least partially magical nature. They are tens of thousands of species of beasts, from small herbivores to truly massive carnivores (for example griffins). Their magical nature allows them to do things that surpass the limits of non-magical biology, while rendering them more susceptible to various types of antimagic (especially antegnite).
Their magical nature also makes them able to evolve further. Many of them develop sentience or even sapience, and each and every one of them can be bend to the will of the Fae (for as long as it's a part of their Court's ecosystem - stronger beasts unaligned with that particular Court might violently resist a Fae's attempt to order them around).
Mortals serving the Courts typically tend to do it because they know no other life. Their ancestors were lost to the Courts often millenia ago. They have, at some point, fully accepted their role as a part of the Court, with many of them revering their Wild Lord as a form of deity. They appear to be cases of said reverence being rewarded with ascendance into faehood at the moment of their death.
They are just as divided depending on the Court in question. A population of one Wild Court might live in permanent utopia of friendly wildlife and controlled environment (to the point of some of them who live in eternal summer forgoing clothes at some point), with their aggresiveness long ago weeded out, while thralls of another Court might live in a world where everything wants to eat them and they persist only by slightly outbreeding the speed at which they die. As one can expect, the different in combat ability between such population will be massive.