Wait, so why did the Archbishop... (SPOILERS)

[Adrasteia]

In all likelihood, Thordan never actually told them of his true intentions beyond, "I want to stop the Dravanians," because that's the only thing that was clearly at the surface. Their likely plan was to get him to kick the door open to siphon the power of the Warring Triad, which he would fail at doing, and thus release the Triad back into the world. They didn't anticipate either his greater ambition, nor the fact that he had the immense power of the second Eye of Nidhogg with him.

[Anonymoose]

In all likelihood, Thordan never actually told them of his true intentions beyond, "I want to stop the Dravanians,"

This was my first impression as well, that Thordan outwardly expressed only that he wanted to end a millennium mired in war against the dragons by becoming the mythological version of Thordan with his Knights Twelve from the scriptures in order to finally end the stalemate with Dravania. That's what I'm going to stick with for now; I think the Ascians would have accepted that without a second thought about what he'd do if he actually accomplished his goal. And who's to say they'd let him? I bet they'd simply offer the dragons just enough power to resist him in a brutal, chaotic struggle that leads to Calamity. The sunset of the Allagan Empire was built on Darkness, and yet they just turned the tide by coaxing Tiamat to summon Bahamut.