Like Inhuman Resource, unlock this achievement by speaking to the owner of the inn in Gran Soren. Select 'Change Vocation'. Now choose any of the classes you see. Keep in mind you have 3 more options than pawns. Available only to the Arisen are the hybrid classes: Assassin, Mystic Knight, and Magick Archer. You will need to be level 10 for any advanced or hybrid classes. Standard and advanced classes cost 1,000 discipline, while hybrid classes cost 1,500. Discipline is earned by killing enemies and you should have acquired more than 1,500 by the time you reach Gran Soren.
For more info on Vocations, go here.