Dark Riders

In the grim days when armies of Daemons besieged Ulthuan, keen-eyed messengers from Nagarythe kept guard for any signs of a daemonic intrusion. These Dark Riders wore cloaks of black feathers, invoking the raven-headed god Nethu to keep them hidden from the eyes of the enemy. During the civil war, the Dark Riders earned a more sinister reputation. They travelled ahead of Nagarythe's hosts, sowing terror and confusion wherever they rode, burning villages and driving their people into the wilds. 1a

To this day, Dark Riders are messengers and pillagers both, carrying tidings between the great cities of Naggaroth, or else riding deep into other lands as harbingers of destruction. Their horses, once pureblood steeds of Nagarythe, are now so twisted by magic and torture that they have become something altogether more malevolent and ravenous. 1a

In battle, Dark Riders take delight in skirting the enemy flanks to attack war machines and cut lines of supply. They revel in the prospect of running down terror-stricken victims, dragging out every moment of wild panic as long as possible before delivering the final heart-seeking strike. 1a

Source

 * Warhammer Armies: Dark Elves (8th Edition) -- pg. 41