Changelings

Changelings are a great mystery. They believe themselves to be a part of the Dragonborn, and certain genetic tests made during the Second Era seem to support that claim. However, their entire species has at this point forgot how their 'natural' form looks like, if they even had any, which makes things much more complicated.
What is known for sure is that the changelings are voluntary shapeshifters capable of free alteration of their own body. They can make themselves appear as anyone they want, regardless of their species, for as long as the target's weight is at least remotely similar.
Of course, 'free alteration' doesn't mean 'fast alteration', as save for very old and powerful changelings they can only make small changes to their born form - or spend weeks on careful modifications of a much larger scope.
That alone would have still made them massively distrusted and persecuted (if not downright exterminated on discovery), however the changelings took a peculiar route. Instead of hiding themselves, they have officially went public with their own existence (during the early Second Era).
They explained what they could do and what they couldn't, and offered to give the imperial government full oversight of themselves, while offering local mages a (quickly proven) mean of magically detecting a changeling. That today can be found is every gate of every city and town, as it can also detect a handful of dangerous shapeshifting daemons.
While it wasn't a quick progress, they were eventually accepted in the general population, although the Empire itself has never truly given up the oversight part.
The changelings themselves are happy living in their own communities, and consider said oversight (which includes regular check-ups of their numbers and the need to gain imperial permits to travel a low price to pay for lack of persecutions, ability to be themselves in the open and a monopoly on a very profitable fleshshaping business.

Basic Information

Anatomy

Nobody knows how changelings looked like originally, including the changelings themselves (the knowledge being apparently lost during the purges of the changelings that occured in the times of the Xylian Dominion.
Elder changelings (about one century long) can de facto rewrite themselves on genetic level to appear as a members of another species in body and genes. The changeling children are born while already 'wearing' the surface species of their mothers.
They lack any DNA of their own (and are instead fueled by a pure magic) so you can't even analyze it. This lead to some suggesting them to be a species of amnesiac daemons or particularly ancient breed of Halfdaemons, or perhaps a hereditary supernatural ability.

Biological Traits

Fleshshaping

Changelings are accomplished healers and fleshshapers. The real problem that makes the 'flesh-alteration magic' not work is that normal mages are utterly incapable of truly understanding the structure of what they are altering.
As a result, any magic of this type will always backfire and kill or maim whoever is altered. The special ability to perceive insides of the body that the changelings possess allows their mages to master the arts of non-theurgy healing and the fleshshaping, however.
As a result, the changeling mages are what passes for the imperial plastic surgeons (although ones that leave their clients appear entirely natural in their new looks), which remains a main income source for their communities.
They can also heal wounds in (almost) instant or - if they are elder changelings - even organize a primitive yet extremely successful organ/limb transplants, with the parts thoroughly altered by them to be completely compatible with the one they are translated to.

Genetics and Reproduction

The changelings can impersonate someone down to a genetic level - which is a process detailed enough that they can actually have a living children with members of species genetically compatible with their surface species. However this doesn't lead to any new changelings being born - the children (following a standard pregnancy, although one that will lock the changeling abilities if they are the ones pregnant for its duration) will be born as 'mundane' members of the surface species.
The actual changeling reproduction occurs once per fifty years, in a number of places whose location is only known to the Imperial Ministry of Internal Affairs and the changelings themselves.
The locations are secret because changelings who congregate in said locations (which means most of their species) are said to temporarily lose most of their cognitive abilities. In fact whatever exactly happens is a complete mystery, that the changelings refuse to talk about.
The protection of the reproduction sites during that time is actually a part of the deal between the changelings and the Empire. The Empire is also aware of what exactly happens there (but maintains its secrecy), and has its officials on-site to register newly born changelings when that finally occur.

Growth Rate & Stages

The changeling' abilities grow stronger as it ages. Between age 1 to 20 they are the Young Changelings, who can only make small surface alterations of their bodies. Between 20 and 100 years they are the Mature Changelings, who can make much faster and precise modifications (that also include the depths of their body, and learn certain degree of the fleshshaping (if they are mages too).
Between 100th and 150th years they are Elder Changelings, who can make extremely quick modifications, including complete switch of their surface species, can master the fleshshaping and are even capable of partially replicate one's brain in order to truly understood their character and even steal some recent memories.

Additional Information

Perception and Sensory Capabilities

The elder changelings (with skills in magic) are capable of shaping flesh of others as if it was their own. However even the young changelings are still extremely talented (and often accomplished) surgeons due to their natural ability to somehow 'see through' every living body and 'feel' its inner workings in exact details - merely on touch (with some concentration and a few moments).
They can instantly locate internal wounds, and can narrow down a poison or a venom coursing in one's wounds to a single compound - once again, merely by touching and spending a while to concentrate on what they feel.
Besides that, the possess the same senses as the creatures they changed into. Some of the eldest changelings are even capable of mixing the bodyparts of several species into a working whole (which they tend to use to give themselves superior senses of a number of species), but this is described as extremely complicated process.