The orcs became increasingly aggressive under the secret control of Gul'dan and his Shadow Council. They constructed massive arenas where the orcs honed their warrior skills in trials of combat and death. During this period, a few clan chieftains spoke out against the growing depravity in their race.
One such chieftain, Durotan of the Frostwolf clan, warned against the orcs' losing themselves to hate and fury. His words fell on deaf ears, however, as stronger chieftains such as Grom Hellscream of the Warsong clan stepped forward to champion the new age of warfare and dominance.
Kil'jaeden knew that the orc clans were almost ready, but he needed to be certain of their ultimate loyalty. In secret, he had the Shadow Council summon Mannoroth the Destructor, the living vessel of destruction and rage. Gul'dan called the clan chieftains together and convinced them that drinking Mannoroth's raging blood would make them utterly invincible.
Led by Grom Hellscream, all the clan chiefs except Durotan drank and thereby sealed their fates as slaves to the Burning Legion. Empowered by Mannoroth's rage, the chieftains unwittingly extended this subjugation to their unsuspecting brethren.
Consumed with the curse of this new bloodlust, the orcs sought to unleash their fury on any who stood before them. Sensing that the time had come, Gul'dan united the warring clans into a single unstoppable Horde.
However, knowing that the various chieftains like Hellscream and Orgrim Doomhammer would vie for overall supremacy, Gul'dan set up a puppet warchief to rule over this new Horde. Blackhand the Destroyer, a particularly depraved and vicious orc warlord, was chosen to be Gul'dan's puppet. Under Blackhand's command, the Horde set out to test itself against the simple draenei.
Over the course of a few months, the Horde eradicated nearly every draenei living on Draenor. Only a scattered handful of survivors managed to evade the orcs' awesome wrath. Flushed with victory, Gul'dan reveled in the Horde's power and might. Still, he knew that without any enemies to fight, the Horde would consume itself with endless infighting in its unstoppable appetite for glorious slaughter.
Kil'jaeden knew that the Horde was finally prepared. The orcs had become the Burning Legion's greatest weapon. The cunning demon shared his knowledge with his waiting master, and Sargeras agreed that the time of his revenge had finally come.