The Black Knight is a breaker of heroes. The latest Black Knight, Amadeus serves as a general and conqueror.
The Black Knight has the ability to control undead, just like the Squire, though the number of undead the Black Knight can control are considerably greater. It also allows the user to manipulate their shadow, fashioning weapons or even using the shadows as extra limbs to carry crossbows in battle. The knight has also used commands empowered by his voice to control soldiers to do his bidding by launching assassinations or suicide attacks on heroes.
Like any martial Name, it grants the user great strength, reflexes and a near-guarantee of winning all but the most massive battles against non-Named. In Amadeus' case it also seems to grant blinding speed but it is unknown whether this is a common feature or unique to him as the Black Knight.