There will probably be a few captive Evokers among the invaders. Some will be there at least half-unwillingly, others have given into the propaganda of the Empire; they actually believe that Evokers need to be controlled to be safe. They don't want to be "freed". Luminal Relic powers, being to some degree of the same source, will not have diminished effect the way torches do in the artificial darkness. I.e. someone capable of generating fire will be more visible in it than they would be in a regular night. 

We'd guess that the Church of Idris and the Imperials will be rather busy going head to head. With contradicting policy and the excuse of religion behind it, they generally won't be able to work together.