Fey and fell creatures existed before the Cataclysm, but like other non-Children races, they possessed no sense of identity or sapience. Fey creatures are the Fey. Fell creatures are Fiends.
During the Cataclysm, Winar granted them a chance--a concentration of magical power would grant them sapience. The power creates an anchor to the land and that anchor, so long as it is sustained by magic, grants self-knowledge, intelligence, and wisdom.
This works in two ways, either many entities gather tightly together in one place, combining their magic to create an anchor, or a single, powerful entity creates the anchor, hosting several less powerful.
The community or host entity creates and sustains the anchor; the anchor sustains the community. Plus, the anchor is inextricably tied to the land.
Any entities encountered away from an anchor are likely to be as mindless as animals and acting more on impulse than thought. The one exception are the sufficiently powerful beings who can wander further from an anchor on their own. Though doing so does mean they are a bit more prone to following whim and instinct over thought, even when they know doing so is a bad idea.
Winar granted sapience so that the fey and fell would know how to protect themselves and could negotiate against those causing the Cataclysm. The primary difference between the Wilds and Wastes is that the fey chose to retreat, while the the fell chose to fight.
Neither the Wilds or Wastes are prone to expansion. The Wastes cannot sustain growth, and the Wilds are far more internally-focused. The Wilds may grow, but only by necessity and always slowly.
Feywilds, home of the fey
When Winar gave her gift, the fey retreated into obscurity to wait out the Cataclysm. The very high concentrations of fey had an almost immediate effect on the forests they'd made home: trees within the feywilds give off magic as a byproduct along with oxygen. This creates a very magic-rich environment that fey have very little reason to leave, save for personal curiosity or internal struggles.
Because the fey chose to retreat and hide, the fey have an abundance of power, but they also have a stronger tie to their anchors. The most powerful fey cannot leave the Wilds without disrupting the ecosystem.
That said, since power is abundant, the wilds don't affect strangers like the wastes do. The impact is on the environment instead. So, though people may enter the wilds and leave with time and memories missing, they do so without being fundamentally changed by the land.
Shadowfells, home of the fiends
When Winar gave her gift, the fell creatures decided to fight back against the people who were destroying the land. This decision changed them, though, cultivating an instinctual and lasting desire for revenge. Since they did not take root, they also developed a stronger need for power. This cultivated a hunger that persists even when ostensibly satisfied.
The Wastes consume magic to persist. It is as necessary to plant-life as carbon dioxide. This makes magic harder to do, since the land will try to consume it before it can take effect.
This need for power also means the Wastes are now, by necessity, more welcoming of outsiders than the Wilds. They need more people just to maintain their anchors, not just fell beings. This is also a consequence from Winar for the fell choice to fight. Fell beings now need and rely on the very same people they desire revenge against for destroying the world.
This enmity, along with the constant need for power, has had an interesting impact on the land. Those who wander into the Wastes for extended periods of time tend to leave changed in some fundamental way. In fact, the Wastes are the origin for many undead creatures and lycanthropes.
Note: The canon background and origin for fiends is dropped entirely. No Blood War, no soul larva, etc. Fiends still need people for their own survival (and hate that they do). Both demons and devils speak Abyssal. Unsurprisingly, Abyssal, Sylvan, and Celestial are very similar linguistically.
When Winar granted her gift, some fey chose not to hide and some fiends chose not to fight. These defectors chose instead to protect the people affected by, but not engaging in the wars.
The people were so thankful, that they worked together to develop a way to become anchors for the celestials. Now, instead of being tied to the land, celestials are tied to bloodlines. They are not bound to protect their bloodline; the ties were a gift, not a pact. Still, most will at least ensure their anchoring bloodline doesn't die out.
Not all celestials have a bloodline anchor, but those that do can serve as an anchor for lesser celestials. Celestials still mourn those who died in the Cataclysm and some even blame themselves still for not doing more. The sinking of Eisiv is supposedly their greatest failure.
For this reason, among others, celestials tend to live in isolated places, such as high up on mountain ranges. Some also choose to live in the Wastes. The only place they are completely unwelcome is the Wilds.
