Eternity itself could be a major theme. It might explore the concept of an ancient sovereign's existence over a long period, perhaps even immortality. Also, there could be themes of leadership and the responsibilities that come with being a sovereign. How the ancient sovereign rules over their domain, makes decisions that affect not only the present but also the future. It could also involve themes of loyalty and betrayal among the subjects and the court of the ancient sovereign as different characters may have different motives regarding the eternal rule of the sovereign.
In an 'ancient sovereign of eternity novel', themes such as legacy and destiny often play important roles. The ancient sovereign's actions and decisions throughout their long existence shape their legacy. Their reign is not just a matter of power in the present but also about what they leave behind for future generations. Destiny could be intertwined with their role as a sovereign, whether they are fated to rule forever or if there are forces that will ultimately end their eternal reign. Additionally, themes of mystery and the unknown might be present, as the concept of an ancient sovereign of eternity has a certain air of the unfathomable.