When I started thinking of how Alex's Ghost Army would come into play, I knew it had to be a situation of no escape. So, I got to thinking, and the first thing I thought of was a person, or more so darkness... Shadow. Think about it, Shadow's never actually been killed before, Alex couldn't kill it, neither could Arthur, Allistor, Lukas, or Vlad... Until last time. But all the other times, everyone thought he was dead too. So, what's not to say he's still alive. If a fragment of him survives, he will too. So say Shadow trapped Alex and a few that she's close to in the shadow realm, which would now be harder to enter without Shadow's consent... For living things. Ghost and spirits could probably go and come freely. And in that realm, ghost would be visible to the others. They've tried pretty much everything, well, they have tried everything... Except for Alex. Who is them, with no other escape options, forced to summon that ghost army.