A Nymph's Return to Nature

Old age is relative in the Feywild, since Time itself is a fickle thing which ebbs and flows, changes its mind as it sees fit. Many think all Fey are immortal, and some might as well be - for a normal human, a being who lives several centuries lives essentially forever. They'll witness so many other lifetimes during their own that at the end of the day, the difference doesn't really matter.
But true immortality is rare in the Feywild. On the other hand, so is Death in its conventional meaning.
The Nymphs are a good example of this. They can live up to 300 years old, and in some rare cases even more, depending on the strength of their will. For centuries they'll look look as young as when they reached maturity, around 20 years of age - the Aurai with their hair eternally flowing in the wind, the Dryads as verdant as ever, flowers along their arms, the Naiads as agile as the water they seem to be made of, the light of the Lampades burning ever bright in the darkest of shadows. But for all, with no exception, there will come a time when they'll feel something shift. It will be subtle at first, like a whisper at the back of their minds, a soft murmur. With time it will become stronger, something they can feel deep in their bodies, running through their veins, a pull, an urge - as if Nature herself is calling them back home. They will know what this means, as all Nymphs do. This phase of their lives is coming to an end, and it is time to begin a new one. For their return to nature doesn't mean death, it is simply a new beginning.
If a Nymph has traveled away from their home they will begin their journey back. Perhaps they will choose a new one for a myriad of reasons; maybe because they have found a place in nature in need of protection, or because their old home has ceased to exist. Once they have chosen this place, since they're beings equally natural and magical, they will become part of it, an essential, and inextricable piece of their existence. This might mean they dissolve into a river or a lake, or that they'll walk into the trunk of a tree and become part of the forest it lives in. Perhaps they will let the wind take them and turn into the breeze that sings softly during Spring. There are as many ways to return to nature as there are Nymphs, and from then on their spirit will still exist as a guardian of that locale. Their memories, feelings, thoughts will take on a different form, but they won't completely disappear. Part of them is still there, it continues on, forever in Nature's embrace, perpetuating their watch as a sliver of what they were protecting from the very beginning of their lives.

Symptoms

This condition will start as a mere feeling, a call from afar, like a longing for something else. There's no given interval of time between when this starts and when ultimately the Nymph returns to nature - it can last from a few weeks, to several months, to several years, depending on many factors. But eventually with time the symptoms will change. The Nymphs would never say they will "worsen", for that assumes this is a bad condition, something terrible that is happening to them, which it isn't. It's merely a sign of the passage of time, and as much a part of their life as all that came before.
The longer the Nymphs either resist their call or simply let it go on, the more it will affect their minds. The calling will become stronger, the urge to finally rest within the fires of a river of lava, or to live as the petals of a flower, becoming louder and louder amid their thoughts, sometimes even overwhelming them. They might even lose control of their minds for a short while, and drift aimlessly, looking for the closest place that most resonates with them.
Eventually the symptoms become physical as well. It will be harder to move. A Dryad's limbs could become even more akin to bark and brambles, making it impossible for the Nymph to move as they did before. Perhaps an Aurai will not be able to control their flight as expertly as before, because even the gentlest of breezes will be able to influence it, to tug at their very essence and beg them to return. A Lampad fire could become increasingly more difficult to control, could even hurt innocent people - something no Nymph could condone unless in extreme circumstances.
There are tales of Nymphs who had such a strong control over their will that they were able to endure these symptoms for years, even decades. Eventually, though, there is no denying that their return must happen. It would be selfish of them to prolong this phase of their lives any more than is necessary. Sometimes it might be necessary, for they may need to deliver on a promise they made, or to protect someone, something, or someplace in need.
But eventually everything ends, and everything changes.
And so they return.

Water Spirit by Haiwei Hou

Nymphs are Nature’s embodiment of physical beauty, and they guard the sacred places of the Feywild. Most nymphs hate evil, and any who would despoil the wilds for any reason. A nymph’s demeanour is wild and mercurial. Like nature, they embody both great beauty and fearsome danger. They can be kind and graceful to mortals who revere the wild places of the world, but are also quick to strike against being who take more than they need or who treat Nature thoughtlessly.
Most nymphs are bound to a glade, a tree, or pool of some kind, but sometimes their sense of duty and protection is overtaken by their extreme curiosity, which makes them wander in search of people and adventure.