Concept: Elves
Hammering out a concept about the place of elves in a fantastical setting:
How elves came to be to begin with depends on who you ask, of course. One popular creation myth describes the Tree of Creation, each of its seeds growing into an elf. Only a very specific number of seeds ever existed, and only a very specific number of elves will ever exist.
Elves are immune to old age. If killed, their souls will reincarnate, growing into a new elf with old memories. Currently, the best guess as to the how is that the soul inhabits an already extant elf and at some point they split into two. The truth lies somewhere between cellular division and ritual self-mutilation.
Elves will adapt to their surroundings on a physiological level. Elves in remote forests will grow nearer to trees (dryads?), elves amongst humans will grow more humanlike. These adaptations even enable elves to sire children the human way, usually resulting in a child of non-elven heritage. But, if the timing is right, an elven soul will inhabit the foetus and a rare half-elf will be born. However, the offspring's elven soul will become tainted, and won't ever return to the pool of elven souls in this world.
For this rather existential threat to elvenkind, most elves despise their half brethren. Some take to extreme measures to preserve elvenkind: They 'proactively' murder those suspected of mingling with other races and use blood magic to then bind thusly released souls to like-minded elves, reducing the risk of further soul corruption. The price is high: The ritual wipes the memory of previous lives from the souls. Purity at the cost of collective memory.