This quest was marked obsolete by Blizzard and cannot be obtained or completed.
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.