With the commonfolk too afraid to speak to you, and Ungust proving unhelpful, Isembard recommends asking the clergy of the nearby church for assistance.
This quest is available if you have completed the quest What Poor People Think.
Starting the Quest
Talk to Isembard in Camp Drybone in Eastern Thanalan to begin the quest.
Speak with Marques
Go east from Isembard and go up the curving wooden ramp. Then go northwest to find Marques. Talk to him to proceed.
Lay the Embalmed Corpse at the Gravesite
Go west into the tunnel. You will find the gravesite at the top. Target it and use the Embalmed Corpse from your Key Items or from the duty list.
Bury the Embalmed Corpse
Check on the Embalmed Corpse to bury him.
Report Back to Marques
Talk to Marques and he will tell you to ask Sister Ourcen about the disappearances.
Speak with Sister Ourcen
Go into the church and talk to Ourcen.
Speak with Isembard
Go southeast to Camp Drybone and talk to Isembard. There will be a cutscene.
Next Main Scenario Quest
After you complete the quest, you will need to be at least level 19 to proceed to the next quest. After you reach level 19, talk to Isembard to accept the next quest: For the Children.