Where did you get this information from? The darkness doesn't dissipate until you kill all the groups of dragons on the map (those that the "eye" shows near the Elven castles). With each time (i.e., with each killed party), the map becomes brighter.
And Nival didn't write anything specific in the mission, only to kill Nicholas :(
...to defeat Nicholas with phoenixes, but to prevent him from resurrecting, you need to leave only one living phoenix at the end.
