After you have shown Roseline's missive to the sentries, Eylgar needs your assistance.
This quest is only available if you started the game in Gridania, you have completed the quest Threat Level Elevated, and have reached level 12.
Starting the Quest
Talk to Eylgar in Bentbranch Meadows in the Central Shroud to start the quest.
Search for Signs
Go southeast from Bentbranch Meadows to the orange circle marked on the map and minimap. In an area with old stone structures, there is a Vulture Egg. It is glowing purple, which means that one or more enemies will appear when you check on it. In this case, two Northern Vultures will attack. Slay them.
Speak with Lothaire
Go southwest from here to find Lothaire in front of a wooden tower. Talk to him to complete the quest.
Next Main Scenario Quest
After completing the quest, you will need to be at least level 13 to proceed to the next quest. After you reach level 13, talk to Lothaire to accept the next quest: A Hearer is Often Late.