Changelings

Changelings are humans who have the ability to shapeshift into other organic beings.
Changeling powers derive from the Soverign Ospra; Ospra had children with many of its priests and passed down its shapeshifting abilities to their offspring.
Whether or not a changeling's child expresses the same gift as their parents, or whether it remains dormant as a recessive seems unpredictable, and at the very least follows no mendelian patterns
In modern times the blood of Ospra is so spread throughout the population that Changelings can seemingly manifest in almost any community.

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Mechanics

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Shapeshifting takes practice. A newcomer may struggle to get the details of a face quite right, an experienced master may may be able to mimic the specific features of someone's pet.
It tends to be uncomfortable for a Changeling to stay in a form other than their native body. different mass? -> pocket dimension? Still writing this out

When a changeling has children, there is a chance their child could inherit their abilities and also be a Changeling. They could also become a Wilding. These two possibilities are not mutually exclusive. The chance of the child inheriting the shapeshifting abilities does not seem to follow any mendelian inheritance patters and is hard to predict. The possibility of the child being a wildling depends on how much the parent has used their shapeshifting abilities.

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Common Misconceptions

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A Changeling's 'true' eyes are yellow/green with reptilian slit irises, and seeing those is a way to tell who is a changeling. This myth has been encouraged and played up by [SPOILER] as a way to let enemies have false confidence that they would have a way to spot changelings.