The Knights of the Ebon Blade is a faction consisting of the renegade death knights that broke free of the Lich King's control after the battle of Light's Hope Chapel (in other words, player-created death knights).
Led by Highlord Darion Mograine, the Knights of the Ebon Blade have allied themselves with the Alliance and the Horde with the help of Highlord Tirion Fordring of the Knights of the Silver Hand, and have pledged to do their part in defeating their former master, the Lich King.
Their main base is Acherus: The Ebon Hold, a former scourge necropolis in the Eastern Plaguelands. (It should be noted that the Knights of the Ebon Blade are not a separate player faction such as the Alliance and Horde. For the purposes of gameplay, player-created death knights still belong to either the Alliance or Horde depending on their race.)