Below are rules for playing Fairy Changelings as PCs. I’ve combined Germanic and Celtic lore on the subject, leaning towards the Irish and Scottish legends. Remember to always be nice and respectful towards the Good Folk.
A Fey replacement for an ordinary human.
For reasons unknown and perhaps incomprehensible to mortals, the Fairy Folk sometimes abduct people (especially young children), and replace them with substitutes called Changelings. Many sages ascribe this behavior to the Fairies’ desire to raise a lovely human child, or perhaps have an attractive human as a lover. Others claim that the Fairies are unable to have children of their own, and so must abduct humans and turn them into Fairies by means of magic. Whatever the motivation, humans will sometimes wake to find their children or loved ones have been replaced with a Fey substitute.
Changelings are sometimes old Fairies that have chosen to end their days among humans. Sometimes the Changeling is actually a young, abandoned Fey. Most strange are those Changelings who are not even natural beings at all, but things constructed from logs or other plants, transformed and brought to weird life by Fairy magic. Whatever the Changeling was in its prior incarnation, the transformation into a Changeling causes it to lose all special qualities of its prior existence, and start again as a 1st Level character.
Changelings appear to be short, skinny humans, but always posses some feature that suggests their supernatural origin. Many look unnaturally old, or possess skin that is unusually pale or greenish in complexion. A number of Changelings, especially female ones, have unusually large eyes; other changeling may have eyes that are too small for their face. Disproportionate heads or limbs, hunchbacks, odd ears, and strangely shaped hands and feet are all possible. In cases where a Changeling replaces a mature person, the Fairies will attempt to make the Changeling appear similar to the original. In all cases, however, a Changeling will have a sickly look about them.
Changelings always appear to be in ill-health, even when they are not. All Changelings eat and drink in large quantities, but never seem to grow any fatter for it.
Surprisingly for Fey, Changelings are not long-lived, and seldom reach more than 40 years of age. Many are killed before that time, victims of accident, violence, or disease.
Changelings can actually breed with humans, and the children produced will be ordinary humans (perhaps with a touch of the Fey about them.)
Psychology & Behavior:
Changelings vary in their outlook and behavior according to their nature. When elderly Feys become Changelings they are often deliberately mischievous and ornery, seeking every opportunity to indulge themselves and make trouble for the duped household that has taken them in. Changelings who had been immature Fae left with human parents will often seem to act out in strange and inappropriate ways, and may refuse to learn how to properly take care of themselves or perform the most basic tasks. Most pathetic are those Changelings who do not know their true nature, and believe themselves to be natural humans. These are often the Changelings that have been made from previously inanimate objects. They may be shocked and horrified to learn their own true natures, and may be driven insane by the revelation.
In no case will a Changeling want to follow what humans consider to be an honest profession, preferring to laze about, sing, and dance whenever possible. Lying comes easily to them. They are naturally attracted to music and art, but also to thievery. Changelings tend to be of dull intellect, but astounding cunning. Likewise, their sickly nature provides them with an excuse to pass boring and tedious tasks off onto others.
Changelings will try to get as much food as they can without actually paying for it. They easily acquire a reputation for being freeloaders. On the other hand, those who appreciate good music are often quite happy to host a Changeling Bard!
Changelings are never of Lawful Alignment, and tend to be Chaotic. Most are Chaotic Neutral. Many are Chaotic Good, and very few are actually Evil.
Hated and feared by most people, almost all Changelings have been subjected to some kind of abuse by their human hosts. Many humans hold to the belief that if they abuse a Changeling enough, the Changeling will re-claimed by the Fey, and the kidnapped person will be returned. Changelings who are old Fey will often have given as good as they got, but the kind who have no memories of their Fae nature will bear deep mental scars. Either way, Changelings almost always find themselves at the extreme margins of human society. About the only way for a Changeling to support herself without criminal behavior is by becoming an entertainer, at which they excel.
Changelings, whether or not they know their own nature, will almost always try to pass themselves off as normal humans. If their true nature is revealed they will generally flee the area. According to some stories, if a Changeling is positively identified they will be forced to return their Fairy brethren, and the human being they are impersonating will be released from captivity.
Changelings, whether they are aware of their own origins or not, will eventually long for the realm of the Fairies. Fey regard Changlings as natural kin, of course. Changelings who visit the Fey will find that their own strange ways are naturally accepted and even celebrated .
Relations With Other Races:
Changelings spend their early years among humans, but humans tend to have extremely poor opinions of Changelings. A Changeling whose true nature is discovered can expect abuse at the hands of humans. Elves, on the other hand, regard Changelings in the same manner as they do any other Fey (i.e. generally positive, but ultimately dependent on the Changeling’s behavior). Dwarves are of a similar mind to humans with regard to Changelings. Halflings regard Changelings with some suspicion, but do not actually hold real animus against them. Half-Orcs often sympathized with Changelings, and understand their troubled childhoods. Gnomes most of all love Changelings, and appreciate their jokes and mischief. Many Changelings find new homes among the Gnomes.
Fae Type: Changelings are Fey, and can use any magic reserved for Fey. Likewise, they are affected by all magic which affects Fey creatures.
Darkvision: Changelings can see in the dark to a distance of 60′. For Changelings who are unaware of their origins, realizing that other humans cannot see in the dark may be their first inkling of the truth about themselves.
Fairy Sight: A Changeling can always see and recognize any other Fey that is invisible, polymorphed, or disguised by an illusion.
Entertainment: All Changelings know how to sing, dance, and use all musical instruments, even if they have never been taught.
Immune To Soul Trapping: The Fairy spirit of a Changeling cannot be Soul Trapped. Neither is a Changeling subject to soul-based effects such as Magic Jar.
Weakness to Iron: Weapons of pure iron are at a +1 to hit and damage when used against Changelings. Changelings will not willingly touch objects made of pure iron.
Great Hunger And Thirst: Changelings must eat and drink twice the normal amount for a human being. Despite their great intake, they are unable to gain weight or muscle mass.
Hatred of Fire: Changelings particularly hate fire, and if they are damaged by fire they must also save versus Charisma or be affected as by a Scare spell. The Scare effect will last for as many rounds as dice of fire damage suffered by the Changeling.
Soulless: Changelings have Fairy spirits, not human souls. After death they are re-incarnated as another Fey. Consequentially, they cannot be Resurrected or Raised from the Dead. On the other hand, they are immune to magic which traps or affects Souls. Because they lack souls, Changelings can never be Clerics or Paladins (but they make excellent Druids).
Revealed By True Sight: Spells such as True Seeing will reveal a Changeling’s true nature and form prior to their change.
Preferred Classes: Bard, Rogue or Illusionist. Changelings cannot be Paladins or Clerics (but they can be Druids).
Bard Abilities Adjustment: +2 on All Abilities
Rogue Abilities Adjustment: +2 on Hide
Illusionist Abilities Adjustment: 1 bonus 1st Level spell per day.
Druid Abilities Adjustment: 1 bonus 1st Level spell from the Illusionist Spell List
Attribute Adjustment: +4 Wisdom, -1 Str, -1 Int, -2 Con. Changelings have a reputation for being thick-headed and sickly, but extraordinarily shrewd and cunning.
Movement: 30 feet
Starting Languages: Common. Changelings also speak the secret Sylvan language of the Fey, without having to have ever been tutored in it.