Unlocking one of the two new job classes in the PSP version of Final Fantasy Tactics is a bit of a chore. The Dark Knight has many pre-requisites to unlock, but once that's done, you'll have a powerful job on your hands.

To unlock the Dark Knight class, you must completely master the Knight and Black Mage classes - that is, learn each of their skills. Also, achieve the eighth level with the Dragoon, Geomancer, Ninja, and Samurai jobs. You must also must have killed twenty enemies in battle to the point that the enemy disappears from the battlefield (three rounds).