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.