Hags

Hags are a curious species that don't really fit neatly into any one category. They are humanoid creatures, but they are not born by biological means and are functionally immortal (though they can still be killed). They're often considered fey creatures, but some types of hags share more in common with the fiends of the Lower Planes than the jovial folk of the Feywild. In addition, while it is often a bit of a mystery as to how fey spirits come about, its well known how hags do procreate: they convert mortal children.
Simply put, a hag forces a child they've "acquired" to imbibe a special potion and then sends them on their way. At first, they may seem fine, but it becomes clear that something is irrevocably wrong as the child continues to grow. Their posture hunches as their skin becomes scaly and discolored; their features become gaunt and their eyes turn cold and leering. What's worse, is that once it becomes clear what is happening to the victim, it's far too late to do anything about it. Only powerful magic, usually that which can only be accessed by nobility or the obscenely rich, could undo a hag's curse once it has set in. And so, the victim is cast out to wander on their own, usually with no recourse but to seek out their now fellow hags that set them on the path in the first place.
Whether the cruelty of their origins fills their souls with malice or the potion simply twists their hearts, hags are generally seen as inherently malevolent beings, though there are rare occurrences where their presence has been considered a boon to the local village. Despite their mischevious and often cruel nature, hags are creatures of their word, and amicable deals can be struck with them, provided the dealing party is willing to stomach the steep price the hags ask in exchange for their services.