One of the problems in covering the history of the orcs is that after the Rise of the Horde period, we've done it already quite a few times. The history of the orcs is the history of the Horde. Just in covering Orgrim Doomhammer's life, we've covered the formation of the Horde to a great extent.
What's interesting when considering the orcs as a people is how they were betrayed by their own virtues. The orc tendency to revere the spirits, their genius at preserving clan individuality yet coming together in times of crisis, their willingness to respect their elders and heed their wisdom -- all of these traits were twisted under first Ner'zhul and then Gul'dan. While Ner'zhul was proud, even arrogant, his initial actions in kindling the war against the draenei were sincere. He believed that the spirit of his dead wife Rulkan had returned to warn him of the draenei threat, accompanied by a "great one" who would teach Ner'zhul new magics to use to protect his people.
No matter Ner'zhul's flaws, it cannot be denied he was sincere. Yes, he hungered for power and respect (even though he was in fact powerful and respected) and yes, he prosecuted the war with the draenei when he really only had the word of Kil'jaeden that the draenei were evil and plotting against the orcs. And yes, Ner'zhul ignored for a time that he was losing the respect of the ancestor spirits and that the elements grew distant from him. He put himself ahead of his role as elder shaman. It cannot and should not be denied. But even in his most aggressive moments, Ner'zhul was neither blind nor a fool. He began to realize that his spiritual advisor, Kil'jaeden, resemble a draenei and hated Velen with a fervor the orc could barely comprehend. He began to wonder why the spirts would not speak to him.
And so he made his way to Oshu'gun.