The death knights of Acherus were the members of the former death knight order under the direct command of the Lich King. They were trained for a specific purpose: the destruction of both the Scarlet Crusade and the Argent Dawn, the last bastions of Light in the Plaguelands.
The death knights of Acherus were replaced by the Host of Suffering with Orbaz Bloodbane, who fled when the Scourge was defeated at Light's Hope, as their leader.