Black Grail Knight
The Black Grail Knights were the resurrected corpses of long dead Grail Knights of Bretonnia. Greater and more infamous than even the Blood Knights they fought alongside, they were among the mightiest undead to have walked the Old World.
Grail Knights are considered the ultimate human warrior, if they fall they are given grand funerals and their souls belong to the Lady of the Lake. This process meant that normally, Grail Knights were one of the few beings who could not be brought into undeath, but some, particularly those whose corpses lay in Mousilion were vulnerable. Even in the tainted lands of Mousillon, only a being of immense power could hope to molest the natural order of The Lady, that being was the ancient Dread King..
Taking up their fortress in Mousilion, the Black Grail Knights would become legendary. So deadly were the warriors, that many didn't even know they were knights, instead thinking that the "Black Grail" was an artifact of immense power and destruction. Many tried to defeat the undead order and all failed .
The Black Grail Knights were eventually destroyed by an army comprised of living Grail Knights and the mercenary forces of the Grudgebringers. With the Black Grail destroyed, they would finally find peace.
Due to the rare circumstances and eventual destruction of the entire Black Grail, it can be assumed that these warriors can no longer exist in the Old World.