Deceivers without par, changelings are known to exist solely based on the occasional deceased specimen recovered over the years. If they have an agenda or are at all widespread, no one knows, and the Changelings themselves aren’t telling.
Outlined in the EbPG, I might be willing to let someone play as a Changeling if they can give me a good reason and are willing to work out that Changeling’s stock personae. The party need not even know you are one. Contact me personally if you really want in on this.