In 263 , after his return , Odaenathus assumed the title King of Kings of the East ( Mlk Mlk ) , and headed to Antioch , the traditional capital of Syria , where he crowned his son Hairan I as coed @-@ King of Kings . The title was a symbol of legitimacy in the East , starting with the Assyrians then the Achaemenids who used it to symbolize their supremacy over all other rulers and was adopted by the Parthian monarchs following their defeat of the Seleucids to legitimize their conquests . The first Sassanian monarch Ardashir I adopted the title following his victory over the Parthians . Odaenathus ' son was crowned with a diadem and a tiara ; the choice of the location was probably meant to express that the Palmyrene monarchs was now the successors of the Seleucid and Iranian rulers who controlled Syria and Mesopotamia in the past .