Death knight is the name of a class shared by several organizations of powerful necromancers. These orders share a few things in common, including riding horses with horned skulls for heads and many of the same abilities.
The death knights of the Second War were powerful undead necromancers created by Gul'dan to replace the warlocks slaughtered by Orgrim Doomhammer. The first and most prominent of the death knights was Teron Gorefiend.
A new order of death knights emerged during the Third War, in service to the Lich King. They were created from living and undead humans (and occasionally other races) who had been granted unholy runeblades, and most were former paladins who had forsaken the Holy Light.
Knights of the Ebon Blade Edit
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 seperate player faction such as the Alliance and Horde. For the purposes of gameplay, player-created death knights still belong to either the Alliance or the Horde depending on their race.)
--Taken from Wowwiki
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