A little thing I noticed.

It looks like a powerful energy is originating from the castle in the trailer, even the clouds above it are spread apart by the force from the beam.

If it's not something just for show in the trailer then I'm sure it'll house some sort of powerful boss or something in it (Possibly Black Mage like mentioned already) The castle appears a second time in the trailer where it seems like they're heading toward it and then monsters suddenly appear as if blocking the path to the castle.

This is the best shot I could get of that part:

 Spoiler

That explains a lot. Maybe the beam is his resurrection?