The Herald of Joyful Tidings

It has been difficult to determine that this creature is indeed one faerie, for it rarely repeats a shape and does not linger long in any place. Only through long and dedicated pursuit, consulting magi, spirits and oracles, was the scholar Lydos of Theios able to put together enough of the picture to be sure of it. The Herald of Joyful Tidings is a faerie that seduces men and women, taking the shape of their preference, and after a whirlwind romance the faerie or its partner become pregnant. Progressing unnaturally fast, a child is born within the month, seemingly human to look upon. The faerie disappears, never to be seen again, leaving its partner with the changeling baby who, if it survives, will usually grow up like a normal child until around the age of ten. Then, some part of its faerie heritage begins to manifest, sometimes as an unearthly appearance, sometimes in magical phenomena. Usually, such children are not accepted by their communities, often being chased out. Almost as common, the babies are abandoned in forests near a town or village - a practice known as being 'left for the faeries' - so that they cannot bring harm to the community. Still, enough of them survive that there are villages whose populations have come over time to contain significant parts of their population with fae ancestry.
The Herald often seems to be drawn to nobility, especially those that are recently married. Needless to say, this can have severe political consequences for a region, and has caused several minor wars. Whether this means any trace of faerie blood exists within noble bloodlines is the kind of thing they would go to great lengths to prevent others learning, and that could cause all kinds of chaos if it were confirmed.
The faerie has no clear motive for its actions; Lydos was able to advance two theories. The first is that the faerie, in the completely-orthogonal-to-human-values way of faeries, thinks it is helping. In this view, Lydos believes the best explanation is that the faerie is aware of the joy children can bring to people, but unaware of, or does not understand, the problems it is causing. In the other, the faerie is malicious, or at best indifferent, and it is simply indulging its own whims while sowing some chaos in the lives of the innocent as a bonus. Under this interpretation, the faerie resembles a demon - the main difference is that a malicious faerie does not care whether it leads its victims into sin, just that it's own goals are accomplished - and it has been mistaken for one before.