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

You need to go through the Dungeon; there will be a portal guarded by Twilight Dragons.

And it's better not to delay.

In the Dungeon, it's best to quickly capture 2 Necropolis Castles and find artifacts for the Prophet. The Prophet will give you 5 Phoenixes for the artifacts. You can use them to attack Nicholas (they should survive), or you can give them to a supporting hero. In the latter case, you need to defeat the scripted dragons as quickly as possible. Otherwise, Nicholas will have legions of dragons.