Priest Seto’s right to the throne
In Yu-Gi-Oh! Millenium World, Atem’s legitimacy to rule is questioned by Aknadin who claims that Seto should be king instead of Atem. Aknadin argues that Priest Seto is of royal blood (as Atem’s cousin) and therefore has a right to the throne.
However, from an ancient egyptian perspective, Seto is not of royal blood and has no more right to be king than Mahad or any of the other priests.
The rules that dictated who could be king and who was part of the royal family were quite different in pharaonic Egypt than what prevails nowadays in modern monarchies.
In Ancient Egypt, the legitimacy to be king derives from the fact that your father was a king. The preference always goes to the children of the latest king to have ruled. So in Yu-Gi-Oh!, only two characters are eligible to be king: Atem and Aknadin as both had a father who ruled Egypt, although Atem is the favoured one out of the two since he is the son of Aknamkanon who ruled last.
From this perspective, Priest Seto is completely out of the line of succession since his father (Aknadin) has never been king. For this same reason, he cannot even be considered a prince of royal blood.
Thus, Atem’s most probable heir, until he had a child of his own, was Aknadin. As Aknadin dies before Atem, that leaves Atem and/or his courtiers to designate a successor. As we all know, Atem chooses Seto which then and only then makes Priest Seto legitimate as pharaoh.
One of the beautiful and fascinating things about this is that this kind of transfer of power actually happened in ancient Egyptian history, in particular during the transition from the 18th to the 19th Dynasty.
Tutankhamun was the son of Akhenaten (the crazy guy who turned Egypt monotheistic for a while) and the last king of his royal bloodline: after Tutankhamun died as a teenager his advisor and commander of the army Horemheb became the ultimate successor after a brief power struggle with the old vizier Ay. Horemheb was of common birth and he never had any children so the 18th Dynasty ended with him.
Horemheb appointed his vizier Paramesse as his successor: this guy (born from a noble military family to a father called Seti and actually serving Horemheb as a High Priest of Set) assumed the throne as Ramesses I and established the 19th Dynasty. His immediate successors were Seti I and Ramesses II.
Long story short, you literally can not make this stuff up. If I recall correctly, Atem is described as an 18th Dynasty king in the manga.
