The Black Knights are a new militia headed by Victor, the ex-leader of the Silver Knights. The troopers are well fitted with the Black Armor, representing their clan. They are responsible for causing the corruption of Silver Land, including draining its water, pollution on its lands, destroying the Silver Knights, and even killing many of Silver Land's natives (especially King Guang).
Victor was drowned by greed, and served under Thanatos' will to create havoc within Silver Land.
The dark organization came to and end when the Grand Chase invaded Victor's Fortress, ultimately destroying their ringmaster.
It was never actually confirmed whether the Black Fairy Queen, Krakos, or Audrey were part of the Black Knights. However, they were responsible for the chaos within Silver Land, so they could have possibly undergone Victor's duties.