I am playing a tengu inquisitor of Magdh. Can anyone give me more insight into what this deity is all about? She sounds Celtic. Are there any Celtic sources I should look at for inspiration?
Sound of a Thousand Screams; Kingmaker part 6, AP #36,
has a chapter on the First World.
Magdh: Capable of seeing farthest into other realms and down the long lines of probability and fate, Magdh is the prophetess and seer of the Eldest. Though her form can change, even splitting into three parts when necessary, the woman known sometimes as the Three most commonly appears as a robed and well-proportioned human female with three faces set equidistantly around her head. Though not independent entities, the three faces each appear to represent a different part of her personality, and in conversation may cut each other off suddenly, as her head swivels to present another face to her audience without adjusting the rest of her body. Those seeking her wisdom and second sight are welcomed according to strange and mysterious guidelines, her required tributes ranging from a child’s shoe to the heart of a star. The knowledge Magdh gleans from her forest pools is often opaque, yet many supplicants believe that the Three sees far more in her divinations than she lets on, and already knows how every story will end—even her own.
For more try searching the First World, Fey, and the Eldest, though anything more than the above might be just a few tidbits.
Courts of the Shadow Fey, Along the Twisted Path, and a few other 3pp might be helpful, it's been awhile since I've seen anything on the Eldest.