Hexwraith

"You can't fight them, and it's a struggle not to flee in horror from them. You just have to hope your commander and his champions really are the holy knights they aspire to be, and that maybe they'll be able to hurt the creatures."

- Alphonse, Bretonnian Man-at-Arms

Hexwraiths, also known as Reaper Knights are ethereal spirits born from the realm of the Underworld, sent forth into the world of the living to hunt down those evil men who would dare to defy their fate and the will of the Death God. The first sightings of the Hexwraiths are shrouded in mystery, but it is said that they are created on the cursed night of Hexensnacht, the first day of the New Year, tearing their way into the mortal realm from the bowels of the Underworld. The Hexwraith's shade-like existence leaves it with a hunger that only the succour of a damned soul can sate.

Once the curse of the Hexwraiths has been laid upon their prey, there can be no escape - the spectral horsemen can hurtle across rivers and pass through mountainsides on their incorporeal steeds without slowing their headlong charge. Ironically, however, in their rush to hunt down these damned souls, they would inevitably become lured into a trap, from which a powerful Necromancer will bind these servants of the Death God into his will.

Hexwraiths are able to move from the realm of spirits to the mortal world and back again at will. They share many similarities with Cairn Wraith, though they are not bound to places of death and grief, but instead are able to roam freely. The scythe-like weapons they use to slay their prey would be lethal enough in the material realm, but because the Hexwraiths shimmer between worlds, their spirit scythes are able to pass through gromril armour or scaled Dragon-hide without hindrance.

A single blow from a spirit scythe can snatch away a mortal's essence whilst leaving his physical form completely unharmed. It is these strange weapons that earn the Hexwraiths their nickname of 'reaper knights', for they harvest the souls of the living just as a farmer reaps his crop. A soul taken by a Hexwraith does not dissipate altogether, but is instead absorbed by the spectre that took it. These dread reapers hence burn with flickering flame; all that is left of the horrified spirits they have stolen from the mortal realm.

The arcane bloodline of the Necrachs were the first to bind Hexwraiths into their armies. Some say they've learnt the art of their summoning from the stolen Book of Arkhan, while others say that the legendary Melkhior were the first to master their control. In recent years, however, they have been seen in Undead armies from all across the known world. When bound into a Vampire's service and commanded by their Hellwraith leaders to ride into battle, whole packs of these apparitions hurtle across the field, plunging headlong through the swiftly dwindling ranks of the foe.

In recent years, the Vampires of Sylvania have learned to bind these creatures of shadow to their service, using them as weapons of war. The sight of a pack of cackling Hexwraiths approaching fast, spirit scythes held high and unnatural soulfires flickering from their eye sockets, is enough to chill the blood of even the most seasoned warrior and send them fleeing. Able to glide through all physical obstacles, whether it would stone, fire or flesh, these Hexwraiths could by simple mortal weapons. It is only with faith and the use of magical spells and items that would have any chance of harming a Hexwraith in combat. It is small wonder that these deathly riders are amongst the most feared of all the minions of the Vampire Counts.

Source

 * Warhammer Armies: Vampire Counts (8th Edition) -- pg. 47