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

Did you get the Phoenixes in exchange for artifacts? And did you kill all the Necropolis dragons and dispel the darkness? Something like that is necessary to prevent Nicholas from being resurrected. I think it's the Phoenixes. What's annoying is that they're not useful for anything else – there are too few of them to make it worthwhile to use them in combat at the end of the mission.