Mortis Engine

The Mortis Engine is a horrific magical artifact that radiates strongly with Dark Magic and kept afloat by a horde of vengeful spirits. Within the Mortis Engine are generally powerful relics that are saturated with strong amounts of Dark Magic, such as the remains of powerful Necromancers and Lich-Lords of ancient times. The Mortis Engines are almost always watched over by deathless attendants known as Corpsemasters, trusted servants of the Vampire Counts who have proven immune to the dire energies that emanate from the relics within.

When the Corpsemaster removes the locks and opens the lead-lined reliquary, the deadly artifact inside can be held aloft, stealing life energy from the enemy and energizing nearby Undead with increased power. The longer a battle rages, the more energies the relic absorbs, and the more powerful it becomes. Mortis Engines can usually be found where the fighting is thickest, drifting ominously near to the battle line where their power is needed most.

However, so redolent with Dark Magic are these artifacts that opening the reliquary is not without risk, for its power can sometimes tear apart the engine itsel£. Indeed, if such a dread relic is ever shattered upon the field of battle, the subsequent release of pure evil has been known to smite everything in the vicinity, living or Undead, in a wave of destructive Dark Magic.

Some reliquaries also carry blasphemous tomes to battle, or scrolls of parchment rumoured to have been penned by Nagash himself. Often times, the winds of magic become nigh uncontrollable when such a fell tome is near. Heavy with evil magics, the pages of these tomes are painstakingly illuminated with such care that their creator's souls have passed into the parchment of human skin that form its pages. In battle, these books can be a boon to the twisted practitioners of necromancy, but also the bane of the reckless and unwary spellcasters that would dare to risk such an event.

Source

 * Warhammer Armies: Vampire Counts (8th Edition) -- pg. 53