The Relentless Horde is the name given to the horsemen of the northern Flanaess when they united under the leadership of Kha-Khan Ogobnuk in (roughly) 320 CY. These were the ancestors of the present day Wolf Nomads and Tiger Nomads.
During the reign of Ogobnuk, the lands east of the Dramidj Ocean and west of the Griff Mountains fell under the Horde's sway, although much of the gains to the east were lost by the time of the Horde's disolution. The Horde failed to conquer Blackmoor, but did cut the northern archbarony off from the south, leaving it isolated. The push eastwards drove the Rovers of the Barren before them, the nation unable to keep pace with the agile nomad raiders. Adapting to the new warfare the Rovers eventually pushed back, costing the Horde its gains in the Rover's lands and and pressing them hard on the Dulsi Plains as well.
The Relentless Horde also made gains in the Sepia Uplands, where they drove out or slew the human inhabitants of the region. The dwarves and gnomish locals proved more stubborn and managed to drive off the nomad invaders, with heavy losses.
When Kha-Khan Ogobnuk finally died in 345 CY the Relentless Horde died with him. The nation split in half, although the two new nations are still bound by their cultural and language tradition.