Changeling
Causes
Reproduction and parenthood are rare, often idiosyncratic processes in the Feywild that vary between its denizens. The Nymph colonies bear children like mortals do; the Eladrin receive their babies as precious gifts from the Tree of All Seasons. Other species weave them from butts, grow them inside flowers, or unearth them from the ground.
But for as long as the treaties that ended the Age of Anxiety hold their magic, the archfey are—invariably and immutably—barren.
Therefore, if the mood to raise a child strikes them, they will travel through a portal or a fey-window, sneak into the home of an infant, and steal them away from their crib. Sometimes they replace the infant with a fey, such as a Goblin. Other times, they form a simulacrim of magic that comforts the parents at first, but withers after a few days. The babies they take in exchange are raised in the Feywild, and not subject to the usual restrictions that mortals must hold themselves to there.
Symptoms
A Soul was not meant to exist for long periods of time in the Feywild. Even an experienced adult Planewalker who spends decades on the plane may notice subtle changes to their senses, such as a tendency towards intense emotions or an enhanced sensitivity to magical effects. Spending one's entire childhood development in the Feywild, though, creates a unique situation in which the changeling's soul learns to accommodate the magical and emotional intensity of their home. Some changelings may display Wildmage-like sorcerous abilities; others may excel in found magic, like that of a Druid, due to their lifelong attunement to high-magic realms.
Affected Groups
Archfey will most frequently steal Half-Elf children. They are drawn to the fey ancestry from their elven blood, but often do not want the commitment of raising an Elf baby for what could well be centuries in Locumtempus Faeroxae. Human babies are also frequent targets, particularly in the settlements near fey-portals. There is no known record of a changeling Tiefling or Aasimar.
Archfey with a particular predisposition towards taking changeling children include Titania, Mab, and the Erlkönig.
Nice article! I like the background you gave to this myth to explain it :D