Now that the beasts have been dealt with, you can examine the correspondence left behind by the heretics.
This quest is available if you have completed the quest In Search of Iceheart.
Starting the Quest
Talk to the Expedition Leader in the Coerthas Western Highlands to begin the quest.
Collect Heretic Epistles
Go into the building. One of the Heretic Epistles is in one of the stables. Then go down the stairs. The remaining two Heretic Epistles are in the southeast corner and the northwest corner of the basement.
Hand the Heretic Epistles to the Expedition Leader
Talk to the Expedition Leader to hand over the Heretic Epistles and complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Alphinaud to accept the next quest: Sounding Out the Amphitheatre.