To unlock Dark Knight, you must reach at least level 50 on any Disciple of War or Magic and finish the final story quest of A Realm Reborn, 'Before the Dawn.' Upon entering Ishgard, accept the quest 'Our End' from an NPC called 'Ishgardian Citizen' in The Pillars (X: 13.2, Y: 8.8) to start the Dark Knight job questline.