The Black Knight

The Black Knight can be found near the eastern end of the Argent Tournament Grounds in Icecrown. He is involved in the neutral quest chain starting with /  and supposedly ending with  upon reaching the rank of Champion in the Argent Tournament. His mount is named Black Knight's Gryphon, but called the Black War Gryphon when riding it.

The Black Knight returned in patch 3.2.0 as the final boss of the 5-man instance the Trial of the Champion within the Crusaders' Coliseum. In the quests and, it's revealed that the Cult of the Damned has resurrected the Black Knight into undeath.

He is slain by players upon the field of battle in broad daylight, by one brave hero and bested upon the field of battle before being brought to justice for his crimes.

Trivia

 * His naming, the naming of his relevant achievements, as well as elements of his encounter, are fairly obvious references to the character of the same name from Monty Python and the Holy Grail.
 * He is statistically the most dangerous NPC in the game, in terms of having killed the most players.