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

What other quests are you referring to? If you don't kill all the spectral dragons or recruit phoenixes into your army, Nicholas will resurrect with a considerable army...

Phoenixes are not mandatory; I completed the game without them. Without them, Nicholas simply won't attack you; instead, he'll transfer his army to the strongest second hero who attacks you! However, without destroying the spectral dragons, you won't be able to kill Nicholas, and you won't be able to complete the game!