Warning This mob usually stands right in front of the book, so make sure that everyone gets the update from the book before you kill him! If you didn't do that, there is another potential way to complete this step without waiting for the respawn. There is another Exarch whose placeholder/assistant is also a herald of Dar back near the center of the Hall of Rites, directly to the south. If that herald is up, he also advances the quest.
You now need to find four keys from the Scaleborn harbingers that spawn in the same room. This is an uncommon quest update.
They are also in a few other locations if it's crowded. Try the Banisher Preparatory ( 170, 60, -50 ) /waypoint 170, 60, -50 (as you leave the exarch room hang a right).
Go back to unlock the book The Chant of the Exarchs.
You must now collect 8 pieces of paper and 8 pieces of chalk.
Slay 8 Droags (any type) to get 8 wings like paper.
Find a droag who can help you translate the map. Take the map to a Scaleborn blasphemer ( 78, 126, 171 ) /waypoint 77.91, 126.14, 170.87 in the Scaleborn Cell. (If you're unable to target him, stand really close to the cell, ensure you have a clear view of him and then use the middle wheel of your mouse to zoom in. You may need to look up to see the dialogue.)
Take a sacrificial blade from the Ceremony Weapon Rack in the Room of Recovery at ( -173, 43, -180 ) /waypoint -173, 43, -180.