The heretics have been aware of the guardsmens' timetables for some time, so you must hurry to Snowcloak to warn Drillemont's knights before it is too late.
This quest is available if you have completed the quest First Blood.
Starting the Quest
Talk to Drillemont in Whitebrim Front in the Coerthas Central Highlands to begin the quest.
Speak with Aymeric
Exit the tower and go southwest to where the orange arrow is pointing in the minimap. Talk to Aymeric to proceed.
Speak with Alphinaud
Go to the quest icon on the map and talk to Alphinaud to proceed. There will be a cutscene.
Speak with the Stalwart Temple Knight
Talk to the Stalwart Temple Knight, and you will unlock the Snowcloak dungeon.
Use the Duty Finder or Duty Support to enter Snowcloak. Please see the Snowcloak Guide for a walkthrough.
After you defeat the dungeon boss and exit the dungeon, there will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Report to Alphinaud
Talk to Alphinaud to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Alphinaud to accept the next quest: For the Greater Good. There will be a cutscene.