This is a mission that can be found under the San d'Orian Nation Missions tab. This mission can be skipped. Trading 3 crystals to the Gate Guard unlocks the next mission.
It's really a simple mission but be aware if you are lower level you will aggro mobs. Basically you talk to an NPC when you enter the zone, move into the zone to click on an ! to get a key item, then return to the prince in North San d'Oria at the Cathedral in the Papal Chambers for a cut scene.