These missions are available from the beginning. You must be allied with San d'Oria to partake in them.
This kingdom of proud Elvaan warriors is situated to the north of the Quon continent and was built on the ashes of a brutal civil war. It has been ruled by a succession of kings from the d'Oraguille royal family for a span of more than five hundred years. The kingdom's capital of San d'Oria is a walled city defended by two orders of knights: the Royal Knights, who patrol the country's borders and seek to expand its territories, and the Temple Knights, responsible for protecting the city itself and maintaining domestic tranquility.
You must have completed the final San d'Oria rank mission, The Heir to the Light, to unlock this achievement. For a complete list of San d'Oria missions, refer to this list.