Striking down the false Warchief
Eitrigg, friend and 'brother' to Tiron Fordring, was born a member of the Blackrock clan. After the Cataclysm, Eitrigg and his last remaining son Ariok decided the time had come to address the Blackrock clan's shameful condition. In the steppes themselves, a triumvirate took control of the remaining Blackrock orcs, General Thorg'izog, Dragon-Lord Neeralak, and High Warlock Xi'lun. These three orcs sought to return the Blackrock clan to power via an ambitious plan to summon and bind a pit lord to their will, then use it as a weapon against the Kingdom of Stormwind starting with neighboring Redridge. They would sweep into the neighboring human kingdom and carve a new Blackrock nation out of the belly of Redridge before any sufficient force could be exerted to stop them.
Now, it's true that Stormwind sent agents to the Burning Steppes as well, but Eitrigg and his son Ariok were directly responsible for the Horde's response to the Blackrock's plan. It was Eitrigg who came up with the plan to turn the three leaders' shortcomings against each other, and Ariok who executed the plan. At the end, all three of the clan's leaders in the Steppes proper lay dead and the remainder of the Blackrock forces retreated into Blackrock Mountain itself. This left the Blackrock clan demoralized and vulnerable to Horde and Alliance agents who repeatedly penetrated this last stronghold of the one powerful and important clan, and eventually, Rend Blackhand died, finally ending the line of Blackhand the Destroyer.
Yet, along with Xi'lun, Neeralak and Thorg'izog, along with Rend died the reason for the Blackrock clan's separation from the rest of the orcs who'd come to Azeroth from Draenor. And around that time, a new warchief was ascending to power, one far more sympathetic to the Blackrocks and their legacy.
The cunning of the son of Hellscream
Having already allowed the Dragonmaw orcs into the Horde, Garrosh was hardly likely to pass up the chance to add yet more battle-hardened orc veterans to his ranks. It must be pointed out that unlike many of the orcs who formed the newer incarnation of the Horde under Thrall during the campaign of liberation that ended with Doomhammer's death in Arathi, the Blackrock orcs have never been in internment camps. Just like the Dragonmaw, the Blackrocks remained free, and if they ever suffered from the same lethargy that left so many orcs shambling in Durnholde and other makeshift prisons, it was sublimated into Rend and Maim's efforts to build a new Horde of their very own. the Dark Horde of Blackrock Mountain may not have won its war with the Dark Iron Dwarves, it may have ended up in service to Nefarian, but it never found itself dazed and looking for answers. They knew who they were, they were Blackrocks.
What's interesting is that, like the Dragonmaw, Garrosh offered amnesty and a place in the Horde to the Blackrock orcs, meaning that Garrosh succeeded in bringing almost all the orcs on Azeroth into the Horde. While the Dragonmaw remained regional in their importance (becoming the backbone of Horde efforts in the Twilight Highlands) the Blackrock have instead come to Orgrimmar, where they've taken up positions in the Kor'kron. Indeed, the new leader of the Kor'kron following Varok Saurfang's withdrawal is Malkorok, a Blackrock orc who renounced his (dead) warchief Rend Blackhand during Garrosh's amnesty for the clan. And that amnesty has had interesting consequences for the Horde as a whole.
Loyal to the orc, not to the Horde
The Blackrocks who serve in the Kor'kron are not loyal to the ideals of Thrall. Indeed, they spent his entire term as Warchief opposed to him and his ways. He sent forces into the Burning Steppes to balk them and kill their chosen leader, Rend Blackhand. Now, with Garrosh having offered them a safe haven, they join the Horde personally loyal to him. They swell the ranks of the Kor'kron with battle-hardened warriors who serve Hellscream, not the Horde. Furthermore, it's not a coincidence that the Dark Shaman who served during the attack on Northwatch Hold appeared after the Blackrock became so big a part of the Kor'kron. The Blackrock served under Nefarian, who was reanimated by his crazed father and set to return to his dark experiments in the Blackwing Descent. During that time, the Blackrocks were allied to the Twilight's Hammer again, as they had been during the Second War before Cho'gall broke off on his own. Indeed, we must remember that Rend and Maim Blackhand assisted their father in making the alliance with the ogres that brought Cho'gall into the Horde in the first place. It's not surprising that the secrets of corrupted shamanism we see unleashed on Northwatch Hold and under Orgrimmar in Ragefire Chasm appear alongside the Blackrock.
Garrosh Hellscream now has a force of personally loyal, completely fanatical warriors at his behest, ones that did not shrink from serving the Black Dragonflight even when the Destroyer of Worlds itself was in command. They have no where else to go. They have no other candidate to lead them, and if they lose this place there is nowhere for them to retreat. They have effectively taken over the Kor'kron, formerly the loyal fists of the Horde, and converted it into a force that serves not the office of Warchief, but the man currently at its head. And they are spying on and even assassinating anyone who dissents from Garrosh's opinions on what is good for the Horde.
Rend Blackhand may have died, but his clan today stands atop the Horde.
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