Cairn Wraith

The Reaper Cometh... with a square base.

The Cairn Wraith is a ghost unit used by both the Vampire Counts in Warhammer Fantasy and the Nighthaunt in Age of Sigmar. In both settings, they're the ghost that looks most similar to a classical depiction of the grim reaper.

Warhammer Fantasy

In the Old World, the Cairn Wraiths are the spirits of the Necromancers and Sorcerers who sought to extend their own lives through Dark Magic. Predictably, this didn't end well for them; they are only just clinging onto existence through sheer will and spite. Now they linger around places of sorrow and fear, feeding off of the residual emotion left over.

Because of how balls to the walls angry at the living they need to be to simply exist, Cairn Wraiths are really hard to rake down. Even the majority of seasoned Light Mages and Excorcists will struggle to banish one.

Age of Sigmar

In AoS, things are a bit more complicated. The wannabe necromancers of the realms now become a Guardian of Souls upon death. But to become a Cairn Wraith? You have to be an especially sick and twisted bastard, even to the likes of the Nighthaunt, the army filled with criminals. That may sound like an exaggeration due to the sheer quantity of them present in the setting, but it's true. When an especially spiteful mass murderer goes to the underworlds of Shyish, they don’t dissipate into the formless spirits or go to their respective afterlives. They cling onto the hatred for life that they nursed as mortals, letting it swallow them whole. Instead of fading away, most aspects of their personality that aren't to do with snuffing out life and/or tearing something to bits get thrown in the garbage. And thus, a Cairn Wraith is created.

Because they were a Nighthaunt unit that predated Soul Wars and 2e, they all used to be lone-wolf style predators. They would hang around near gravesites and other deathly places, snatching up anyone unlucky enough to wander in. This changed after the Necroquake, where it's explicitly said that bone daddy bound ALL of them into his service with an enormous necromantic monolith known as the Great Cairnoch. This leaves most Cairn Wraiths across the realms either roped into a Procession or into the service of a particularly strong Vampire or Necromancer. Regardless of who's holding the keys to their cage, the Cairn Wraiths don't particularly care, as long as they're allowed to reap their due.

Soulbound

It's rare to see Soulbound Cairn Wraiths, even within purely Death bindings. Because they're all raging murderous ghost psychos, it's hard to persuade them not to butcher absolutely every living being within reach. If a Binding has a Cairn Wraith stuck with it, then that usually means that the circumstances are dire indeed. As such, most bindings that find themselves saddled with a Cairn Wraith find themselves with less of a teammate and more of a living (or unliving) weapon to lug around, point at an enemy and let it rip whilst minimising unnecessary casualties.

Nonetheless, they aren't entirely emotionless - otherwise they wouldn't be included as a player option. Some Cairn Wraiths have certain predilections towards killing a certain quarry. If a binding is landed with a Wraith that enjoys seeing Nurgle cultists go pop when they're ripped apart, or one that likes chasing down Skaven as they flee, then they have something to actually work with.