From the preview:
Called the Dread Horsemen by some and Doomriders by others, the Death Knights are ever at the forefront of Waiqar’s army, riding from the mists to cut down the living. Donning ancient armor and aided by the speed of their terrible steeds, these mounted warriors demonstrate a skill in battle unmatched by lesser undead. Unencumbered by the weakness and doubt brought on by the fear of death, these cavalry descend upon the countryside of Terrinoth to herald the coming of Waiqar and his undying legion.
Serving as the unholy counterparts to the mounted knights of mortal realms, the Death Knights are capable of fighting on long past the limits of mortal men. Possessing none of the weaknesses of living flesh, Death Knights know nothing of hunger, fatigue, or fear. Similarly, the vampiric steeds of these dread riders are capable of speed and stamina far beyond even the greatest stallion, particularly when they have caught the scent of fear or blood in the wind. When the Equius Cumir must feed, they are known to devour the flesh of man and Elf alike, sating their terrible hunger on the corpse-laden fields of battle.