Northern Wind

Northern Wind is, in an essence, a splinter organization of the Imperial Magic Guild that has broken off the main body three centuries ago. However, the ideological roots of it date way back into the Second Era, with the Northern Wind inspired by numerous writers advocating for taking advantage of the baseline human moldability in order to 'perfect' their forms by genetic engineering, hereditary mentalsculpting, eugenics and various magical procedures. Some go even further in search of inspiration, back to writers of the Helian League.
The Northern Wind sees the general population fear of such procedures (as everyone's still pretty traumatized by what the Xylian Dominion was doing) as backwards and obsolete. In fact, it has a rather signature feeling of superiority towards the imperial citizens as a whole, which would most likely increase further if they even get to achieve their goals. The end plan of the Northern Wind is to pretty much do what the Dominion did, but even more. Start by changing themselves into a genetically (and magically-enhanced) superior caste, and (perhaps) eventually change everyone else into an inferior servile caste.
It is known for its numerous violations of the imperial laws, including illegal experimentation on sapient species and attacks on the Imperial Magic Guild in order to seize their assets (and that's if you do not count their occasional connection to the Council of Shadows, with the Northern Wind often selling their body-altering abilities in exchange for thins they need for their experimentations). Plus, of course, the fact that they are unregistered magic users.
The problem that the modern Empire has to face is that the Northern Wind seems to be getting closer and closer to achieving their goals, at least if the quality of their bodily enhancements is anything to go on. It used to be an occasionally amusing distraction in the past that was mostly overlooked (save for the Imperial Magic Guild going after them), but now it's growing into a menace. Today the Northern Wind is practically the only group of people in the Grand Empire with detailed knowledge of genetics - and the ability to apply it. Of course, they are also very humanocentric and occasionally turn out to be less intelligent than they think they are, but this changes little in the greater scheme of things.

Structure

The organizational structure of the Northern Wind is more or less resembling the one of the Imperial Magic Guild, but it's slightly less egalitarian, and internally reorganized in order to work well for an organization that's supposed to remain a secret. There is also a singular headquarters for a region (typically located in the Lost Lands near one of the provinces), and numerous regional cells that are supposed to gather resources or information for said headquarters, typically through some intermediaries.
Assaulting such headquarters is typically a massive pain in the ass for even the Grand Empire's forces. In modern times it is considered to be unadvisable to launch an assault on them without a Chosen One or their equals in slightly less overpowered opponents. There are even cases when the Grand Empire knows the Northern Wind's lair - and the Wind knows that the Empire knows - but the regular assault is simple impossible due to lack of firepower in the area or the surrounding Lost Lands (sometimes you simply can't send an army somewhere - which is a problem when you need an army in said somewhere).
Each cell is run by a handful of the high-ranked members of the Wind that went through particularly expansive set of enhancements. It's an unofficial rule that they are all mages, however the degree of said enhancements means that they are actually capable of measuring up to many martial artists in close-quarters. How high you are ranked as a whole depends on the degree of enhancements - genetic, surgical and magical. The less human you are, the higher your standing.

Military

The Numbers are the regular troopers of the Northern Wind, most of them in fact captives after thorough brainwashing and some minor enhancements. They are altered by the Wind to be mindlessly obedient, and completely devoid of personality and past. The name comes from the fact that those 'puppets' aren't even given names, only numbers (which is a part of their individuality-suppresing mentalsculpt).
They are disciplined and they never flee from the battlefield. Even when captured alive, they are simply wiped clean too much to be able to restore them to their old selves, resulting in mercy kill being preferred form of 'treatment'. On the other hand, they are simply not particularly powerful. Although sometimes quite numerous (and they can double as servants to keep the bases clean).
They also have some dedicated units who receive more enhancements and some specific indoctrination, while still remaining part of the Numbers in general treatment. They serve some dedicated combat roles, from field surgeons, through magicians to well-trained strike teams tasked with assasinations or swift elimination of some powerful enemies out in the wild.
The Evolved are the end-result of the Northern Wind's focus on improvement. The Evolved are the group's answer to the imperial Warforged. In practise, all elder members of the Northern Wind are Evolved, but there are several steps of that. The higher you go, the rarer the individuals who managed to attempt - and survive - the enhancement procedures. However the result are incredible - high ranked Evolved are one of the few combatants in Karadia who are capable of holding their ground (even if temporarily) against Chosen Ones, and as a whole are playing in the same league as magisters or martial masters.
The Evolved can still be tricked. Which is harder than in case of normal humans, but it's still about their only weakness. Whether they are mages or not, they are simply incredibly dangerous. However, unlike the Numbers, they simply lack quantity due to insane amount of resources needed to create them and the risks of the procedure failing spectacularly (and lethally).