What do you have to do to unlock the dark knight job in FFXI?
Answers:
First, your character must be at least Lv.30 in any job (warrior or thief preferable). Head to the Bastok mines area, and look for someone named Gumbah on the upper map, near the weapons shop icon. Talk to him until you get a cutscene. Now head over to Palborough mines; you can get a map in Bastok.
Head for square H-8 on your map, towards the river. You should be able to find a jetty with a boat. Once you get in the boat, you will meet with dark knight Zeid, who will give you a sword called the Chaosbringer. You are then told to kill 100 enemies with it (grab a pen and paper though, the game won't count for you). The sword is VERY slow to use, but you should be fine if you stick to killing low-level enemies.
Once you have 100, change to your Lv.30 job if you haven't already, then head into Beadeaux via Pashow marshes. You will meet with Zeid once again, and afterwards you will be able to choose the Dark Knight job!
Oh, and hang on to the Chaosbringer. You need it later.