Draeca:
Princep's points are all valid. I'll observe, though, that the heirs will be in hiding now and that the more straightfoward magic for contacting them (like sending) will likely be blocked by various protection magic. Contacting them might mean going to them directly but it would be worth trying indirect means first if possible. For example, the heir might be blocked but it is not practical to block all who know him so maybe we could scry a known bodyguard or companion and communicate through them.
Princep's thought on the flying fortress near Lyko is interesting. If the heir is holed up there, it might be worth trying to go to them there although that could be an 'alamo' kind of stand. Draeca would be up for that, though.
Both of those lines would benefit from a cyrstal ball to scry out people likely to be in either place. If no one in the party has one, we should see if Ikon will assist us in that matter.
Draeca thinks that if we can find some information that gives us a lead on the flying fortresses, we should pursue that angle. It would be interesting if we could defeat a Mottul party that carried with it the means to fly the fortresses. That is, if Mottul can fly multiple fortresses and if they are then going to attack the fortresses to get them and then make them fly, we could potential get the secret to flying them from the Mottul raider sent to seize the fortress. Aside from just hanging out in one of these in the hopes Mottul will attack, Draeca is not sure how to pursue that, though.
Failing that, rather than chasing heirs all over, she is inclined to go to Fina and fight the armies of Mottul on the ground. But don't forget that Mottul armies are full of undead and she has a thing about undead.
