egypt-museum:

“Although the Egyptian sources equate the deceased king with several deities, there is a clear and constant emphasis throughout most of Egyptian history on the association with the king of the netherworld god Osiris. This was doubtless because the role of kingship fitted the Osiride mythology particularly well. 

 Every pharaoh ceased to function as the earthly Horus -and son of Osiris- upon death and was identified by virtue of death with the deceased Osiris. He thus stood as predecessor in relation to the next living king as the mythical Osiris did to Horus. According to this symbolic metaphor, by becoming one with Osiris the dead king also became ruler of the afterlife region switching realms, as it were, from rule over the living to rule over the dead.”

― The Complete Gods and Goddesses of Ancient Egypt, by Richard Wilkinson