Amber Crown

The Amber Crown is a faction born of betrayal, slavery and copious amount of racial hatred. At some point in the past, its predecessors were simple elves (typically from the lands of the elven empire) who had the distinct bad luck of living during the time when the human armies have finally laid the empire low. It was collapsing all around them, and so they decided to seek refuge among the Fae of the Wild Courts.
It was, it has to be said right away, a horrible idea. Making decisions while desperate leads to such ideas being made into reality way too often. Sure, some of the Courts simply converted the new arrivals en masse into the wild elves, some - especially those who for some reason detested the elves or simply the Wild Courts that were rather malevolent - were much more creative. The former elven nobility had it particularly hard, with many fae apparently enjoying breaking the haughty elves in many ways. Unfortunately (or fortunately), making decisions on the basis of petty pleasure is also a common source of horrible ideas.
Two decades later the elves born from such unions turned out to be something else that both sides expect. They were a completely new elven subspecies (later on dubbed the amber elves). Naturally-born - and extremely long living - mages of nature, with those descending from highborn elves (who were typically 'entertained' by True Fae) actually inheriting the Fae's ability to control the nature. In case of the former elven royalty (who were typically handpicked by the Wild Lords), the results were even more extreme.
Soon approximately 1/5 of the Wild Courts fell to violent rebellions from the inside, with the descendants of elven royalty murdering their Wild Lord's parents and usurping their domains with the powers they inherited. The result was the birth of the Amber Crown, a millenia-long power (that has survived the Second Era mostly by militant isolationism) that seeks to push the humans out of Karadia, wipe out the remaining Fae, and reestablish the ancient elven empire that they see themselves as successors of.

Structure

The Amber Crown is an essentially feudal state spread throughout the Lost Lands in a way not very different from how the imperial provinces work like. There is an Empress (although nobody from the Grand Empire has ever saw her, despite her being apparently as old as the Amber Crown itself), there are numerous kings and queens ruling over the dominions that they usurped from the Fae millenia ago (although sometimes those are ruled by the descendants of the original usurpers). Then you have their advisory councils that double as de facto governments, various noble houses, numerous soldiers and mages, and finally the general population (plus some non-elven slaves that they took during their wars).
What differentiates the Amber Crown from the Grand Empire is that it managed to practically tame the Lost Lands. The amber elf royalty exerts the same power over their domain as the Wild Lords they usurped. The plantlife, the animals, the insects and the climate - it's all in their control, including the local beasts (only fae are exterminated on-sight). Because of that, launching an assault on their domain is considered to be extremely dangerous, if not downright suicidal. Especially as behind the forests are the walls of their fortresses, guarded by the spellcasters and warriors who are, as a whole, way above the human average.
To lay siege on such a fortress while surviving the relentless harrowing attacks from everything that lives beyond their walls is a feat achieved only by a few. Even the Second Empire decided to limit itself to thoroughly massacring the Crown's forces in the field until they finally retreated into their domains and stayed for the the rest of the Era. Attacking the fortress-cities of the amber elves directly was considered to be a waste of life that was simply not worth the effort.

Military

The Leaves Numberless is the best available translation of the elven name for their regular military. They are unfallingly loyal, they are tougher and faster than natural elves, and have a natural gift for magic. They also have a lot of time to train, as with their degree of fae-ness in the blood they will live for several centuries. They also receive good quality equipment.
They are, in fact, one of the mortal armed forces that is considered to be a match for the regular forces of the Imperial Army, at least depending on the place where the battle happens. When in place with lively nature, they will hold a slight advantage, while on an open plain the Imperial Army will typically triumph.
The Amber Hearts of the Amber Crown is NOT to be underestimated. They are equivalent of the more elite formations of the Imperial Army, especially due to the length of their service - some of them can be centuries old. These gave them unmatched time to train. Add the natural talent for magic and the strengthening of their body by the fae blood and you receive something absolutely terrific.
Their only weakness is their low number. It's rare for the Amber Crown domain to be able to field more than a thousand of them, both because you need certain level of strength of your fae blood and because each of them is given an equipment that is as functional as it's beautiful (and thus, time-consuming and costly).
The Nobles of the Amber Crown are special caste due to their ability to control the beasts through their fae blood. They are even less numerous than the Amber Hearts (who are typically recruited from what can be considered the upper middle strata of the Crowns' society), with few domains of the Amber Crown having more than a dozen houses.
However, every noble is an absolute terror of powerful magic and physical might (not to mention practical immortality, which means that some of them still remember the initial rebellion against the Fae), who can often be seen doing impossible feats such as using wild griffins as mounts. As a result, they are considered to be a prime elimination target for elite squads of elite adventurers wherever they are spotted.

Religion

Today the Amber Crown is the sole remaining bastion of the ancient elven religion, at least in Karadia. This means, in essence, lots of deities (who are at best at the level of the imperial demigods or lesser gods), who receive proper sacrifices and grant boons in exchange, but their interest in the matters of morality or personal feelings of their worshippers is extremely limited.
Next to them are the royals of the Amber Crown, now considered to be akin to living gods by the rest of the Amber Crown. Even the nobles do not dare to plot against them - every plant out there can repeat their words to the local royal, and this means being instantly smitten down. It's a horrifying tyranny, in a way, one that everyone was conditioned to accept and gave it vestiges of religion.