I never really put much thought into it, but in hindsight I assume that if Kyros set all this up so an Archon would awaken in the Tiers it would be so they would "adopt" the Tiers as their home and get rid of some problem Archons. Once the Tiers are conquered, Kyros has nowhere else to invade, which means he wouldn't have anything to keep the attention off his own tyrannical actions, which is clearly already a problem considering the Forge-Bound are discontent already, and a number of untrustworthy or unstable Archons would be left without distraction. We know Kyros had at least three armies, so why send the two that are polar opposites and commanded by generals that hate each other when he had a larger army (mentioned at the end of the game) that was more united and would have been able to conquer the Tiers without invoking so much rebellion in the natives? I think it's because Kyros needed to have someone capable of opposing his Empire so he could have a way to paint himself as more benevolent by "protecting" his people from the "savage barbarians" of the Tiers and he didn't want to have Graven Ashe or Nerat around anymore.
Throughout the Conquest, Kyros had Tunon assign his most promising Fatebinder to oversee and mediate these two armies to plant seeds of doubt and frustration in the Fatebinder's mind (after all, how could Kyros truly be a great and capable ruler if his generals are such fools?) and then, when the oathbreakers rebelled and both Nerat and Ashe were in a position that they could be trapped within, Kyros wrote an Edict to kill them off and had Tunon send the same Fatebinder to read the Edict, worded in such a way that it would allow a frustrated and desperate Fatebinder to ally with the rebels instead of try to get the two armies to work together, still satisfy the conditions of the Edict, and potentially awaken their powers as an Archon. I think the issue is that Kyros didn't anticipate the new Archon would have the power to proclaim their own Edicts and everything went downhill once they gained the ability. His solution would have been to declare what was simply a free for all between the Archons in the Tiers to buy him time to get the rest of his armies in position to put down the new Archon before they started to test their powers should the others fail. In this scenario, Kyros then also makes the mistake of assuming that the new Archon can't issue an Edict on any major territory in his empire because his own Edicts have to be read within the location they would impact, so when the player declares an Edict on his capital Kyros isn't prepared for the damage and has to call back his armies to maintain order.
Basically, if Kyros planned all this, I tend to view the ending as "Kyros' plan went too well"