Mages and Changelings appear, at least on the surface, to be polar opposites. The former are fundamentally human, despite their power, and the latter are no longer such. The former can reshape reality on a whim, while the latter has been shaped by a reality wholly unlike our own. Even their methods of becoming more than human are dramatically different; Mages awaken to a deeper understanding of reality and how it functions, while Changelings are stolen from their homes to be taken to otherworldly realms where they are warped and changed into something no longer wholly human.
Despite all these differences however, the two supernatural species share many similarities. Both are capable of altering reality around them, either through sheer willpower or through "Contracts" struck with reality that allow them to bend the rules. Both are worried about the things that lurk outside of our reality, be they called the Gentry, the Kindly Ones, the Goetic Daemons, the True Fae, the Ancient Ones, or any of a thousand other names. Above all, both are seeking the truth behind what has caused the walls between this world and the ones "outside" to weaken and what this might mean.
Who are the Gentry?
The nature of he creatures that rule over Arcadia is a hotly debated topic amongst scholarly Changelings. Some say the Gentry are Aliens from outside our reality, others argue that they are creatures that existed before our reality and built Arcadia as a dark mockery of our world, and still others argue that they are the distilled essence of mankind's dreams and nightmares. The truth is unknown and many argue unknowable, but many study them nonetheless. What little bit these researchers have learned is that the Gentry are essentially immortal, as they can only be killed by each other, and they need human beings for some reason. To what end the Gentry take people is also unknown, as the Gentry can reshape Arcadia at will and create any creature they desire in the form of Hobgoblins. What's more, most tasks they set their captured mortals to are typically menial and could be done by themselves or their Hobgoblin creations, but the fact that they do abduct humans and warp them into Changelings is undeniable.
What is a Changeling?
At its most basic level, a Changeling is simply a person who was abducted from their world and taken to one of the many realms of Arcadia. What and where exactly Arcadia is has to substantial debate between Changeling scholars and those Mages who are aware of such a place, but one thing is certain: it is a place both outside and completely separate from reality as human beings understand it. What is also certain is that these realms are not a place well suited to human survival and only through direct intervention of the creatures that inhabit these realms can a human being exist there unaltered. As most of these inhuman denizens are just as (if not more) inhuman than their homeworlds, they often do not know or care to know how a human being could survive in these realms and instead they force the adaptation of their subjects to the realm as quickly as possible. Even after the Changeling escapes back to our world, they find themselves forever changed by their experience and it is this change that defines them as Changelings. Fortunately, Changelings discover that they are hidden from mortal eyes by a mystical Mask that hides their now otherworldly nature.
What is the Mask?
The Mask is what keeps Changelings from being noticed as being anything other than human and seem to be a subconscious enchantment they place on themselves. The Mask resembles what the Changeling looked like in the time before their abduction by the Gentry, though some hints of their inhuman nature leak through and distort the image. Antlers might manifest as a strange hairstyle, goat-like eyes will appear as simply very dark human eyes on the Mask, and stone skin resembles dry and leathery skin. The Mask also hides those who are inhumanly large or small (though it is rare that Giants are larger than 8-9 feet and even Dwarves are rarely smaller than 4 feet) by shrinking or expanding their image to be closer to human average. Most importantly, the Mask simply makes a Changeling appear as little more than an eccentric mortal, allowing them to interact with human beings as they would anyone else. Of course, the Mask can always be removed if the Changeling directly chooses to reveal themselves, but even then their identity is protected. Once the awe and horror of seeing the Changeling's true form wears off, the Mask returns and the memories of mortals cloud. They remember seeing something strange, but can't quite place what it was...
What is a Fetch?
A Fetch is a special form of Hobgoblin which is sometimes created by the Gentry when they abduct a mortal. The Fetch is a creature made of stray materials present at where the mortal was taken, typically stones sticks, leaves, trash and other debris easily at hand. The Gentry shapes this debris into the rough shape of a human being and enchants it with life. Through this enchantment the Fetch not only becomes mobile and sentient, but is also granted both the memories of the abducted mortal and a Mask that matches said mortal exactly. Most Fetches are completely unaware that they are anything other than the mortal they made to mimic, indeed many are horrified at the concept of being anything but who they are made to be. They blend into the mortal's life and convinces both family and friends that nothing is amiss. Why the Gentry care to create such deceptions, as most mortals could never find the stolen person if they wanted to, and why some choose not to is unknown. What is known is that the Fetch is the largest obstacle in the way of a Changeling returning to their mortal life upon returning, and is indeed the reason that many do not bother. Instead, many Changelings devote themselves to their fellows who also escaped Arcadia.