Before the creation of Idyll and Hell, upon the death of their body, a good soul could become a nymph, instead of reincarnating as others did.
These nymphs would then become guardians of the land, often attached to a particular feature such as a tree, a source, a fall, a lake, a river...
Nymphs are designated protectors of the land and take this duty very seriously. Their main enemy is the forces of evil, but they will take action against any mortal they judge dangerous, especially if their local is threatened in any way.
They sometimes also help passersby in need.
Upon becoming a nymph, the soul could choose any appearance desired, even though some chose something similar to their prior life, many created peculiar bodies suiting their fancy.
Even though most of them chose a gendered look, often but not always the same as in life, they are actually asexual and cannot reproduce.
All nymphs are physically bound to the chosen location and cannot stray far and are unwilling to do so. They want to protect the local nature.
There are a few exceptions who go on to explore the world, but this is rare.
The Nymph World
As very magical beings, over time they created themselves a semi parallel world which is only visible to those who have a sort of second sight.
This world is physically part of the prime plane, but at the same time a bit out of phase. One could be walking through it without knowing if one can't see it.
It is populated with many wonderful beings besides nymphs. All require second sight to see them, but a few can chose to be seen.
These beings are truly immortal. Their physical form can be killed, but then their soul starts wandering until it settles in another region which soothes it and creates a new body in tune with its surroundings.
Even though it is immortal, it's memory isn't. It will remember the last century clearly, but the preceding two or three will be hazier and it will get foggier the further back it tries to remember. Nothing will be left beyond a millennium.
After the death of a nymph body, the soul forgets everything. That's how sometimes there are "new" nymphs in the world.