The Dark Knight is one of the most powerful and difficult jobs to unlock. It requires mastering several other jobs and meeting specific kill count requirements. To unlock Dark Knight, a unit must master Knight, Black Mage, Ninja, and Geomancer, and have a total of 20 unit kills (friendly or enemy) in their career.
The Dark Knight's 'Darkness' abilities are incredibly strong, dealing massive physical damage and often having area-of-effect properties. However, many of these abilities consume the Dark Knight's own HP. This job is best suited for units with high physical attack and HP, and often paired with a strong healing secondary ability or 'Auto-Potion'.