Inner Sea Gods says that one of Pharasma's servitors is a unique linnorm named Birthed-in-Sorrow, and gives some basic details on her abilities (channel positive energy, animate objects). Do we know anything more about this creature from any other source? Motivations? Backstory? Stats would be nice, because I think my player is going to be fighting her soon.
I'm pretty sure what's in Inner Sea God is all there is.
You probably want to look at a linnorm of 18 HD or less as your basis for stating up Birthed-in-Sorrow. I.e., something that can be pulled in through a greater planar ally spell. Here's a list of the linnorms and their stat blocks - link
A fjord or gare linnorm - or other water-related linnorm - might be thematically appropriate.