Zamorak is extremely powerful, but Zaros was a god. Not just any god, but a being of ancient power that eclipsed any other divine being on this world. The Staff of Armadyl must be a weapon of awesome potential - far more so than its weakling namesake - to have enabled such a feat. Imagine what I could do with such an artefact!
Armadyl returned the staff to Temple of Ikov shortly after Zaros's defeat, and I am unable to penetrate the Temple's defences. I have, however, been able to research its history and capabilities in some detail through interrogation of the Armadylean faithful, among other methods. I believe the staff predates Armadyl, and may even have been a causal factor in his ascension to divinity! According to the Armadylean legends, the ability granted Armadyl by the staff is to comprehend and manipulate the relationships between the nature of things. Zamorak must have had no inkling of this, or he wouldn't have been stabbing things with it.
In the course of my investigations I encountered a number of ghosts around the empire, including one that claimed to be our old friend General Viggora. The ghosts told a strange tale of Zamorak's acquisition of the staff, mixing personal testimony with whatever dreams and imaginings ghosts experience in their tormented unrest. The strangest claim, consistent across all of them, was that Zaros had placed them in this state in a final act of vengeance for their part in his downfall. As much as I may be glad to see Zaros gone, I knew him well enough to find the claim implausible. I must believe they had something to do with the acquisition of the staff, though, or how would they even know of its existence?
My theory is that when Zamorak crudely invoked the staff's power by the simple expedient of stabbing it into Zaros, some of the power that was released fed back through the staff into everything it had a recent connection with, especially those who had been in physical proximity to it. How or why it trapped them in that dead state I do not know. As for Viggora, he was a pathetic shadow of the mighty chieftain I had once counted as an ally, wallowing in imagined self-pity and his own delusions. When I returned to squeeze more information from him some years later, he had retained no knowledge of our previous meeting and responded to me as a stranger once again.