The knights of the hundred year war.

Apparently this quest is only available after completing the quest 'Legendary Magician'.

Before accepting this quest, you should go to the archives in Marseille and search the archeology shelves until you find 'Graveyard Map', search/unlock rank 9. (note: rank 7 unlock is sufficient to open) Apparently rank 12 archeology is needed for the map to appear. The map tells you where to find the Knight's Crest, which is a necessary item for completing this quest.

Once you have the crest, you need only to talk to the Adventure GM to complete the quest.

Hope this helps!