Dark Riders are Dark Elf light cavalry.
They are known as the heralds of Naggaroth, for they are ofte found scouting ahead of the army. Their Dark Steeds are very fast, ensuring that the Druchii riders can easily charge the enemy with their spears, then pull away when they are threatened. However, they don't need to charge the enemy to cause destruction as they are also armed with repeater crossbows and combined with the famed Elven accuracy and swiftness of their steeds, this creates a deadly combination. 
In the time before Ulthuan was invaded adn devestated by a Daemon invasion, Nagarythe born scouts rode across the continent on swift steeds as messengers and scouts, looking for signs of incursions by the neverborn creatures of Chaos. They inoked the god Nerthu, wearing black feathered cloaks hoping that the raven god would hide them from their enemies. [2a]
Much later, during the Sundering they were found riding beyond the army of Nagarythe, bringing terror and confusion as they burnt settlements and drove the surivivors into the wilds. They still perform this task but also act as messengers between the great cities of Naggaroth whilst in battle they errupt from flanking rides to slaughter war machine crews or devestate their supply lines, enjoying the panic of their fleeing victims. [2a]
- 1: Games Workshop Official Site
- 2: Warhammer Armies: Dark Elves (8th Edition)