Rothe asks you to assist Thierremont, who may have taken on too much work for one person.
This quest is available if you have completed the quest Work in Progress.
Starting the Quest
Talk to Rothe in the Coerthas Western Highlands to begin the quest.
Speak with Thierremont
Go north and attune to the Aetheryte if you haven't already done so, then go west or east from the Aetheryte along the red tile. Go down the stairs, then go north to where the orange arrow is pointing in the minimap. Talk to Thierremont to proceed.
The locations of Deepeyes will be marked on the minimap. Kill them until you have obtained three bristles.
Deliver the Bristles
Go to the quest icon on the map and talk to Thierremont to hand over the Dark Bristles and complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Thierremont to accept the next quest: From on High.