We don't know exactly how events played out before Garrosh's arrival. It seems clear, however, that one big difference between the two timelines is that in this one, Ner'zhul's wife Rulkan was not dead when the Burning Legion started their manipulations. In our timeline, Kil'jaeden first appeared to Ner'zhul in the form of his dead wife's spirit. Here, clearly that wouldn't work, as Rulkan was very much alive.
In this Draenor, it seems that Kil'jaeden realized that it would simply be easier to tempt t he avaricious, power hungry Gul'dan directly with offers of power rather than working through Ner'zhul. And it nearly worked - Gul'dan drank the blood of Mannoroth and offered the power of the blood to the orcish clans directly, after whipping up hostility towards the draenei, all without Ner'zhul's help. Ner'zhul's reaction to his former student's actions is not known - perhaps they horrified him, or perhaps he simply had other, more pressing issues to deal with. For Shadowmoon was the home of the Dark Star, and Ner'zhul had a heavy responsibility to protect his people from its influence.
The rise of the Iron Horde and Gul'dan's disgrace didn't immediately concern Ner'zhul, but the arrival in Shadowmoon of the new Warchief of the Iron Horde certainly did. Because while the Dark Portal was under construction, while Grommash Hellscream took the plan that Gul'dan had for conquest and re-imagined it, and decided all orcs must be part of the war machine he was creating. He visited the Shadowmoon with Garrosh at his side and offered Ner'zhul a simple choice. Either join the Iron Horde or die, and your people die with you. Furthermore, joining wasn't sufficient - it wasn't enough that Ner'zhul commit his people to a war they didn't particularly want to fight. Grommash wasn't impressed with the Shadowmoon Clan's tendency towards astrology and reverence for the dead. Of what good was shamanism and vague mummery compared to the industrial might of the burgeoning Iron Horde? If Ner'zhul couldn't produce tangible results, real power, his people would still be destroyed.
There was a source of power available to Ner'zhul. It was, however, one he was forbidden to make use of. When the draenei first arrived on the world they would name Draenor, their vessel, the Naaru dimension ship Genedar was badly damaged, and in the chaos of its crash landing into Nagrand one of the Naaru, K'ara (potentially the namesake for the Temple of Karabor) was injured and entered into the regenerative void phase of Naaru existence. In the process of the crash, K'ara was ejected from the ship, and floated high above Shadowmoon Valley. Seething with void power, this presence was detected by the astologically minded Shadowmoon, who at first made use of this strange new power they could access and which they'd never experienced before.
However, they soon came to realize the power came at a terrible price. The shadowy magics of the Void that the Dark Star granted caused those that tapped this power to grow corrupt, arrogant, and in time, disrespectful of the ancestor spirits that the Shadowmoon venerated.
Ner'zhul knew this, of course. It was emblazoned on the Ancestral Totem of his people, an artifact that all shamans of the Shadowmoon Clan were bound to obey. It became one of the most sacred of strictures, for it was bound up in the preservation of the sanctity of the ancestor spirits themselves, which was seen as the very soul of Shadowmoon culture. To be a Shadowmoon orc was to be protective of the ancestor spirits, to commune with them to gain wisdom, to revere and protect and guide them. His wife Rulkan counseled defiance to Grommash and the Iron Horde - they couldn't give him what he wanted and remain true to their most sacred beliefs. They couldn't do this thing and remain Shadowmoon.
But Ner'zhul, faced with the destruction of his people, and believing they stood no chance against the Iron Horde war machine, did not defy Grommash as the Frostwolves did. Perhaps he believed that the Frostwolves would eventually fall - after all, while they are protected by their harsh climate and treacherous terrain, in time the Iron Horde would simply overwhelm them. And Shadowmoon lacks the natural defenses of inhospitable Frostfire. Whatever his reasoning was, Ner'zhul chose to break the ancestral totem, to defy the strictures of his ancestors. To tap into the power of the Dark Star, and use that power on the behalf of the Iron Horde.
In the years since he made that fateful decision, Ner'zhul has learned much of the Void. Rulkan abandoned him, taking as many of her clan as would go into exile. Ner'zhul, once so devoted to his wife, no longer cared. His contemplative nature seemed gone, warped by the Void he was steeping himself and his clan in - he now seemed to embrace the path to conquest of the Iron Horde, and sought even greater power - to draw the Dark Star to the surface of Draenor and unleash it on the draenei, wiping out Karabor in the process. The Ner'zhul of old had no particular animus against the draenei, but the one who drew upon the Void gleefully kidnapped them to serve as slaves and sacrifices to empower the Dark Star.
And worst of all, he succeeded. He summoned the Dark Star itself to Shadowmoon Valley and unleashed its power.
In the end, although the cost was enormous (the life of Velen, who had guided the draenei for 25,000 years) this rebounded to the detriment of the Iron Horde, as Velen's sacrifice purified the Dark Star and unleashed its power not on behalf of their invasion of Karabor, but against it. So in a way, Ner'zhul helped defeat the Iron Horde. This went over about as well as you can imagine. While the Iron Horde couldn't immediately strike against Ner'zhul (they'd just lost a massive invasion force to the draenei) the Void-touched shaman knew the consequences for failure. He retreated into the Shadowmoon Burial Grounds with his most powerful servants and plumbed the depths of the Void, seeking a means to protect his clan from Hellscream's vengeance.
He didn't focus his attention carefully enough, as it turned out. While he communed with the Void, the angry forces of the Alliance and Horde sent agents to infiltrate his hiding place, strike down his servants, and eliminate him before he could either return to the Iron Horde as a force to be reckoned with or find an even more devastating weapon than the Dark Star had been. Ner'zhul paid the price for abandoning his clan's most sacred principles - but it remains to be seen if the Void is finished with him. After all, Ner'zhul had unleashed a power that defiles the spirits of the dead, and raises them as unholy abominations, the walking dead.
It's possible it was this power that the orcs of our timeline used when Gul'dan developed the death knight - it's telling that Teron'gor, one of Gul'dan's most faithful servants in both timelines, became the first death knight using secrets wrested from the Temple of Karabor after it became the Black Temple. Was K'ara the origin of the Reliquary of Souls? The fact that Ner'zhul became capable of warping and defiling the dead into unlife as soon as he touched the Void implies that the Dark Star's power was used to twist the teachings of the Deathspeakers - teachings Gul'dan would have been privy to, as he was once a Shadowmoon orc and a shaman of the clan.
And therefore, it's possible we haven't seen the last of Ner'zhul. In this world, this timeline, he may rise as a force of pure shadow, an undead monstrosity given unholy force and unlife not by a malevolent demon but by his own works - a kind of monarch of unlife, a ruler of the dead.
A king of liches. Darion Mograine certainly seems to think there's a reason to study this further.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.