Corpse Cart

Corpse Carts act as magnets to Dark Magic, drawing their power from the land itself and using said power in either bolstering the combat abilities of nearby Undead or to raise them back into battle. Sometimes a Corpse Cart is hung with a great bell, the clapper of which is a fell lodestone of eldritch provenance. When necromantic magic is thick in the air, the bell tolls and ripples of Dark Magic emanate from the Corpse Cart. Under the influence of this ominous knell, the dead are drawn back together and cadavers stagger to their feet. Other Corpse Carts are lit by braziers that burn with supernatural flames. The smoke from these balefires contains particles of warpstone that have the horrific ability of driving enemy wizards temporarily insane.

The first Corpse Carts came about during the year of 1111 IC, when the Chaos Moon, Morrslieb, showered down great chunks of Warpstone meteorites upon the lands of Sylvania. The citizens of Sylvania during that time were wracked with the horrors of the Black Plague, a powerful disease unleashed by the vermins of the Skaven Under-Empire in an effort to weaken the Empire of Man before their inevitable surface invasions. When fragments of warpstone were scattered all across Sylvania, the bodies of these victims became imbued with large amounts of Dark Magic.

The warpstone meteors would in time also affected the plague wagons that carried the thousands of dead plague victims to the charnel pits. Bodies fused together into writhing Undead masses, and the drivers of the carts changed into strange, withered creatures. The infamous Necromancer, Frederick Vanhal, formed an army of the Undead from the newly risen, using his Dark Magic to bind the Corpse Carts to his will. Infused with warpstone and driven forth by Vanhal's will, the Corpse Carts acted as a conduit for the Necromancer's magic. When Vanhal's apprentice turned on his master and killed him, the Corpse Carts were no longer controlled. Without Vanhal's guiding influence, the devilish chariots dispersed to all corners of Sylvania.

Though most were hunted down and put to the torch, for over a thousand years, the remaining Corpse Carts have wandered along the back roads and dirt tracks of Sylvania, carrying death and misery with them like a shroud. On dark nights, they enter ramshackle villages, stirring the dead in their graves and filling the dreams of slumbering peasants with visions of rot and death. These strange constructs are highly valued by all Vampires, for Undead creatures near a Corpse Cart will be imbued with large amounts of Dark Magic, giving the Undead the ability to fight with an almost uncommon frenetic vigor and savagry.

Source

 * Warhammer Armies: Vampire Counts (8th Edition) -- pg. 40