Dark Knights were formerly a type of enemy you could encounter in an early version of Mount&Blade. They wore the finest armor in Calradia, the Black Armor. Due to their superior equipment, they posed too great a threat, not only to the player, but other lords as well. This contributed to their eventual removal from the game.
They were removed from the game between version 0.800 to version 0.894. The Black Knights can still be found in a mod for Mount&Blade: Warband called "Native Expansion" and are extremely difficult to fight.
Their armor still exists in the game's files, and you can re-enable it by editing the game.
Suprisingly, Dark Knights did not wear black helmets, instead they wore guard helmets.