Reikenor

One of the most trusted Nighthaunt lieutenants, Reikenor the Grimhailer was once a sorcerer-king in Hysh. Because Hyshians are known for trying to perform monumental tasks involving lots of hubris, he decided to try and unbend the very workings of death itself. You can imagine how Nagash took this. In the afterlife, Reikenor hunts not for the secrets of mortality, but for those that were once like him; petty mages, those that unbind necromantic rituals, and especially Necromancers that don't serve Nagash.

He swoops down from above on a winged Nightmare with a scythe, all grim reaper looking, and steals their souls. These souls are then used for his own unique spell called the Wraithstorm, which involves a cyclone of these souls being let out to attack their former comrades.

In Broken Realms: Be'lakor he got his own subfaction, which buffs up Chainrasps and Glaivewraith Stalkers.