Ranger - "Higher Vampire" [5th Campaign, 5th Mission]

Can you tell me why it's necessary to kill ALL spectral dragons? Shouldn't you only kill those that guard the Chains of War?

Where did you get that information? Until you kill all groups of dragons on the map (the ones that the "eye" shows near the Elven castles), the darkness doesn't dissipate. With each time (i.e., with each killed group), the map becomes brighter.