Verdauga received gifts from Yogg-Saron, which drove him partially into the control of the Old God. He then spread the "gifts" like a virus to those around him. Luckily, most of Verdauga's friends were resilient to the trinkets which whispered madness into their minds. There are three reason why Verdauga might have been easily influenced by the Old God. The first is that Verdauga is getting older and more senile, and was perhaps part mad to begin with, If indeed Verdauga was mad in the first place, then the madness of Yogg-Saron perhaps had a different reaction upon him. Secondly, Verdauga has visited the Emerald Dream - which is being corrupted by an Old God (though it is unknown if that God is Yogg-Saron) therefore the Tauren might have been more vulnerable to corruption from Old Gods. Lastly, the whispers of the Old Gods are known to be powerful in coercing any being - they can corrupt far greater beings then Verdauga.
It is interesting to note that while Verdauga became mad and revered Yogg-Saron he was never violent or benevolent. He would speak jibberish and try and convince others to follow Yogg-Saron so that "Azeroth could be reborn in darkness". He would also become posessed at times by a strange Iron Dwarf. Despite all this Verdauga was a very "sane" mad person and he brought no harm to anyone. If he was challenged by someone regarding his beliefs he would merely shout at them that they would soon see his words had meaning and that to resist would invoke doom by Yogg-Saron, and then he would run off. He had almost a split personalty - his faith in the Earthmother fought hard to resist the whisperings of the Old God.
Verdauga returned to Kalimdor and out of Yogg-Saron's reach and took his guild (Braan Firetotem included) to the master's glaive in Darkshore - a skull of an Old God. He spoke of reawakening the being, which was imprisoned in death by a titan sword. He spoke calmly about raising the ancient gods, those ignorant to the malevolent beings may have taken his words as wisdom not madness. He was challenged by a member of his guild at the master's glaive but Verdauga fled instead - the whispers leading him to Blackfathom Depths. Within the depths dwelled Twilight Cultists - Old God loyalists.
Verdauga willingly offered himself to them and sat upon one of the thrones of the "six seats". The Cultists within sought to carry out the will of Yogg-Saron and corrupt and therefore create six new completely dedicated Old God followers. Verdauga was one.. and the eyes would follow him to rescue him after receiving whispers of his impending doom and the other seats would be taken. The eyes, led by Braan entered the depths - dedicated to Verdauga and desperately seeking to end his serene corruption. At the beginning of the month, Verdauga had communed with the spirits who had told him of an old face returning - evidently this face was of Harb the warrior, as he appeared that night and helped defeat the Cultists and rescue Verdauga. Without Harb's aid, Verdauga and the rest of the eyes who ventured into the depths would have been consumed by the will of the Old Gods entirely. Braan and Mariel were strong members, but no match for the cultists without Harb.
After the ritual was disrupted Verdauga awakened - shed of his dark garb that he would infamously wear when preaching about Yogg-Saron or revealing his madness. He sat casually drinking tea, greeted his friends in a calm yet confused manner and thanked them for saving him. They left the depths and returned to the surface, Verdauga did not act strangely or openly reveal a dedication to Yogg-Saron for some time afterwards. But the corruption of the Old Gods runs deep, and the old mind of an elder Tauren can not so easily escape the grasp of Yogg-Saron.
As loyal to Verdauga as his followers were, and as much as they strived to restore him, they could handle no more. They could and would not follow him down his dark path. As hard as their leader had tried to corrupt them, it was their absence from Northrend that ensured their safety from the dark God of the north. From their homeland of Kalimdor the Thousand Eyes had experienced the gradual and painful fall of their leader with his every homecoming. Finally they left him and without his followers he was a mad lone bull with no real influence and all were wary of him. His mission to bring Yogg-Saron's corruption to Kalimdor had failed, and the Old God would slowly release Verdauga now that he was no longer of any use, a shattered tribal leader. Perhaps if Yogg-Saron had not been subdued by the forces in the north he would have sought out yet another leader in Kalimdor to corrupt in order to extend his influence into the southern continent.
Through the self-sacrifice of Possum Fenhide to the spirits, Verd regained his sanity. The love the Taureness displayed was unlike any other, and she gave her life to restore this once great leader, though he would never be restored to his former glory and nor would the Eyes.
When the Eyes departed Verdauga became Verdure in order to start again as a new Druid. As Verdure he was a lone wanderer and fought the evils in the north, seeking redemption. By the time he met his death he had redeemed himself in the eyes of some, and did not leave the world the insane, raging bull he was known for but a calm and wise Druid that he should have always been. Sad is a tale of the fall of the wise, but even the greatest mind can succumb to the whispers from beneath the earth... as history tells of Neltharion.