Black Knight

"Go not to the mounds on moonless nights,
 * seek not the dead to rise
 * Risk not the wrath that sleep and dreams,
 * heed the words of the wise.."

- Chorus from a traditional Empire Peasant Song

Black Knights are the horrific reanimated corpses of long-dead tribal Cheiftains or feudal Knights who were raised from their rest by Necromancers and Vampires to bolser the ranks of his Undead army. In the times before the Empire, there were few domesticated horses, and horsemen were exceedingly rare. In most tribes, a steed was a symbol of wealth and status. So it was often that only a chieftain and his closest warriors would ride mounted into battle, the wealthiest of their number clad in crude iron plate and carrying stout shields. When these early knights died, their horses were ritually killed and buried in the barrows alongside their masters, to carry them in the afterlife.

Many centuries later, the Vampire Counts are known to summon forth the gruesome remains of those ancient knights in order to bolster the masses of lesser Undead minions under their control. As a Vampire stalks through barrows and mausoleums, Dark Magic swirls invisibly around him like a cloak, probing and penetrating the cracked and overgrown porticos of each resting place and saturating the bones of the armoured corpses within. Without the blessings of Morr, these corprses are at the mercy of the Vampire.

A thousand years of dust shifts and dissipates as the parchment-dry skeletal corpses twitch and sit upright. In a morbid mockery of their old lives, these Black Knights stir into motion. Alongside them, the skeletal remains of their steeds jerk to unlife, twisted by magic into hellish mockeries of the noble beasts they once were. Outside the barrow, the Vampire splits open the resting place of his new servants with a deafening crack. At his word, fully formed Black Knights ride pell-mell from the tumble down ruins of their tombs, whole units of warrior horsemen arrayed in the corroded armour of a bygone age

When called into battle, Black Knights would often form into squadrons of powerful shock cavalry. During their mortal lives, these unearthly knights used weapons bearing enchantments of destruction. Though corrupted by the patina of the ages, these tools of slaughter are no less potent than when the wielder was a man of flesh and blood. It is said that one pierced by a Black Knight's lance or sword never recovers. Their steeds are trapped in a strange half-life by rituals that have bound them to their riders. There have been reports of Black Knights galloping straight through the rubble of ruined cities, or even charging across the surface of a lake without leaving so much as a ripple. These unliving cavaliers crash into the ranks of their living enemies, spitting their foes on lances wreathed in cold flame, and lashing out with heavy swords older than the Empire itself.

Source

 * Warhammer Armies: Vampire Count (8th Edition) -- pg. 39