The Scourge is the original name for the Tiefling race.
The Infernal Empire, which once dominated the known world, was largely established by armies of the Scourge. It is said that the tieflings were betrayed by their masters and in response rebelled. They considered their vengeance complete only after they had dismantled the empire beyond all hope of reconstruction.
Some of the Scourge even now consider vengeance against the unjust to be a sort of divine purpose of their race.