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.

Chain

Morthis Whisperwing → The Ward of Wakening → Waking the Sleeper → No Mere Dream → Return to Morthis Whisperwing → To the Evergrove → The Book of the Raven → Eyes in the Sky → To Catch A Sparrowhawk → The Raven Stones → The Eagle's Essence → The Falcon's Essence → The Hawk's Essence → Return to Cenarion Refuge → Chasing the Moonstone → Vanquish the Raven God