These missions are available from the beginning. You must be allied with Windurst to partake in them.
Tribes of Tarutaru from the sweeping grasslands and neighboring islands of southern Mindartia united to form this league of states. It is governed by a council of representatives from the five ministries of magic, legacies of the five founding tribes. Their decisions are based on the prophecies of the mystic Star Sibyl who resides in the fabled Heavens Tower. In one section of the city live the Mithra, a race of natural hunters whose skills have brought a new source of wealth to Windurst. The capital, Windurst City, was razed in the war twenty years ago, but this prominent center of learning has since been restored to its former glory.
You must have completed the final Windurst rank mission, Moon Reading, to unlock this achievement. For a complete list of Windurst missions, refer to this list.