In WHFB Hexwraiths were riders of the underworld, rising from places suffused with the energies of death and going upon ghostly hunts for the souls of those who have earned their ire and or malign attention. They can pass through all obstacles, from vast stretches of ocean, to seemingly impassable mountains, all to hunt down those they have marked for reaping. It is however their single mindedness in the hunt that they are sometimes trapped and dominated into service by strong willed necromancers for their undead armies. The vampire counts in particular make extensive use of them as outriders and shock cavalry, cutting vast swaths in the enemies forces with their intangible scythes, ignoring armor, cover, people in between to hack and slash both soul and flesh.
Age of Sigmar
In AoS, they are the souls of murderous Knights, stuck in the Nagash patented, trapped in a endless torment where only the destruction of life and all things living can slightly alleviate™. They serve much of the same role as their WHFB counterparts, as shock cavalry and soul reapers, riding along side the other phantasmal horrors of the Nighthaunt.
On the Tabletop
|Leaders:||Cairn Wraith - Dreadblade Harrow - Guardian of Souls - Knight of Shrouds - Krulghast Cruciator - Lord Executioner - Scriptor Mortis - Spirit Torment - Tomb Banshee|
|Troops:||Bladegheist Revenant - Chainrasp - Craventhrone Guard - Dreadscythe Harridan - Glaivewraith Stalker - Grimghast Reaper - Myrmourn Banshee - Black Coach - Hexwraiths - Spirit Host|