As the imperial army hunted down the male members of the Hindonpon royal family, the first thing Neonidas did was set a date for the ceremony where all the lords of the kingdom, whether great or small, would swear fealty to emperor.
Neonidas then officially declared the two princesses of the Hindonpon kingdom as his concubines and took the former queen as his slave. This could easily be seen as a highly disrespectful move.
In conquests like this, princesses and other high nobles were usually gifted to the king—or, in Neonidas's case, the emperor. But Neonidas didn't do that. The reason he took these two princesses as concubines was that he didn't want Emperor Gredor, his future enemy, to gain more power.
With these two new concubines, Neonidas could lay claim to the Hindonpon throne, but even if he didn't, his son would naturally become the ruler of the Hindonpon kingdom—or, in the future, the duchy of Hindonpon.