Use an Aether Ray Eye with your Seer's Stone to reveal the raven cult ghosts. Search the Vortex Pinnacle for the spirit of Sai'kkal the Elder and question him about the book of the raven, then return to Arthorn Windsong.
From what you've told me, Morthis's suspicions seem plausible, but we need confirmation.
In his research, Timeon has uncovered clues that a "book of the raven" served as an ancient scripture to the cult.
The arakkoa spirits will not reveal themselves to your eyes. Take this stone with you to the high peaks in the western part of Blade's Edge, to the south of Raven's Wood, obtain an aether ray eye, and combine them to augment your sight. Search for the ghost Timeon identified as Sai'kkal the Elder.
Also, you get: 7 40
<Arthorn looks at you hopefully.>
Were you able to track down any information about the book?
Isn't it amazing that even among arakkoa, the cultists were considered heretics? That the traditional priests hated them enough to destroy their holy text is incredible! We must get our hands on that book, <name>.
It's the key to unraveling the mystery of the raven god and preventing him from taking over the Emerald Dream.
Perhaps Morthis will finally agree to take me as a student if we succeed!