The forces of Exris are mass-produced suits of armor imbued with souls summoned by Exris. Though they are not the strongest monsters out there, they have a few tricks that make them dangerous as an army. They don't have to eat or drink so tiring out is never a problem and seige tactics are useless against them. Additionally, their hollow bodies allow them to carry things easily.
What if someone stored a way stronger monster inside of them? Now THAT would be unexpect-- oh wait that already happened.