By the time the Toa-Hunter War happened in Metru Nui, Teridax had been assigned to watch over the city for some 76,000 years. Although I'm aware that by this time he was conspiring to depose of Mata Nui, allowing the Dark hunters to capture the city would be of no benefit to him (in fact quite the opposite; without a stable Metru Nui the Matoran Universe falls into disorder), and helping the toa would have lent credence to his-benevolent front. So where was Makuta Teridax?
And as a follow up, what was his relationship with Turaga Dume prior to impersonating him? Did Duma struggle with sharing power, as he did with the Turaga of Mata Nui, or was he not even aware that Teridax monitored the City of Legends?