A changeling, very interesting. There are a million different places we could fit this in!

A deathless changeling could be a very formidable foe, indeed. 

The first question that comes to mind - is the shapeshifting biological, or is it magical in nature? Canonwise we could go either way.

Biological - Vechern has extremely high biodiversity, precipitated by frequent impact events in its history and a need for quick evolution. If we're going the biological route, I'd love to get into the history of this species. What was the environment like that led to the development of shapeshifting as an ability? An extreme need to blend in. What would motivate this? Perhaps changelings have been hunted to near extinction by some sort of superpredator. 

Magical - This would be a little harder to work, because the magic in this series isn't something that is easily controlled or channeled. I lean toward, perhaps inside the Great Sky Rift, there is life, these changeling creatures, and one of them (or several) manage to board the [i]Overture[/i] while it is in flight through the rift and they assume the form of people on the expedition. This could lead to some fun. 

Either way, having a deathless shapeshifter is something that could dramatically change the calculus for the players and politically for all of the different big players in the world of the RP, so I'm all for it. I think it's great.