Drillemont asks you to go to the Boulder Downs to assist in searching for the heretics before they can attempt an ambush.
This quest is available if you have completed the quest Dark Words, Dark Deeds.
Starting the Quest
Talk to Drillemont in Whitebrim Front in the Coerthas Central Highlands to begin the quest.
Rendevous with the House Durendaire Knights
Exit the tower and go south to where the orange arrow is pointing in the minimap. Dismount and enter the glowing purple circle. Defeat the Hateful Harrier with the staff, then the one with the lance, then the one with the sword and shield. Next, defeat the Hateful Harrier with the lance, followed by the one with the sword and shield.
Search the Heretic's Effects
Check on the sparkling Fallen Heretic's Effects.
Deliver the Heretic's Parchment
Go north to where the orange arrow is pointing in the minimap. Enter the tower and go to the top floor. Talk to Drillemont to hand over the Brittle Parchment and complete the quest. There will be a cutscene.
Next Main Scenario Quest
After you complete the quest, talk to Drillemont to accept the next quest: The Path of the Righteous.