Azarathius' magical heatwave tricked the sea savages, he managed to sneak your army and his pyromancers right to their doorstep. The ancestral city is in shambles, but not destroyed yet.
The gates open up and let out a horde of warriors, you must keep them from reaching Azarathius, the spell must be completed, the city must be destroyed.
You successfully thwarted the assault and allowed Azarathius to finish his incantation. He hurls a massive fireball toward the city, upons contact everything is bathed in blinding light, it seems as if Dyn'Tur has been disintegrated.