Compared to building the Two Rivers Empire from scratch, converting the Dar Empire was like a dream! The facilities were all there if a bit singed or damaged.
The cities already existed and crops were already set up in a cycle. There were resources already waiting to be collected and harvested as well as new ones to be planted in the case of herbs.
All in all, 15 years seemed too short but it was nothing compared to the demons. Haim was emperor. He had a few upstarts at first but his power crushed them brutally. The new city lords tried to rebel and found a damned dragon and silver roc smashing their dreams of independence to pieces, but very few died.
After the first 5 years, the focus of the other empires did help unify the demons and aether children.