According to a tale fabricated by a spirit who was none other than Teron Gorefiend, the remaining death knights knelt before him and offered their lives as a sign of loyalty, after which Teron killed every one of them. From the fallen death knights rose the Ghostriders of Karabor. After the ritual, Teron killed himself. They currently patrol the perimeter of Shadowmoon Valley, bearing Gorefiend's Truncheon. They can only be seen by equipping the Spectrecles provided for the Teron Gorefiend quest chain or the helms provided as rewards for it.
The Ghostriders follow a large circular route using the available roads in the interior of Shadowmoon Valley. Additionally, the Ghostriders typically ride their route counter-clockwise. Since traveling the same direction would mean they'd have to be overtaken, the quickest method for locating them, therefore is to fly/ride the route clock-wise.
This story seems to contradict the fact that Turalyon killed Teron in battle, making it impossible for him to have committed suicide. This leaves one to believe that Teron lied about that episode, as he invented an entire story for heroes to believe and resurrect him. The Ghostriders themselves are therefore of uncertain origin if the tale of their beginning is a lie, but they are most certainly death knights, and likely former servants of Teron Gorefiend. More about this is on the Teron Gorefiend article.