You shout at the soldiers to spark amber torches while you stride to where the witches are. The Thracian witch, Aurelia, is still under the effects of the aether and will be of no use to you here, but the other two could be. You know they would never do so willingly under normal circumstances, but their lives are as much on the line here as everyone else's. Death by burrowers is a bad way to go. Taking them by the collars, you explain as much.