How did the myth of Nergal and Ereshkigal explain the rule of the underworld?

In Mesopotamian religion, the myth of Nergal and Ereshkigal serves to explain the establishment of a dual monarchy over the underworld.

Originally, Ereshkigal reigned alone as the queen of the dead, presiding over a sealed, grim realm from which no subject could escape. According to the myth, Nergal descended to the underworld escorted by demons. Upon his arrival, he confronted Ereshkigal and threatened to decapitate her. To save herself, Ereshkigal agreed to take Nergal as her husband. Through this union, Nergal obtained kingship over the underworld, sharing authority with Ereshkigal.123

This narrative accounts for the transition from Ereshkigal’s solitary rule to a shared governance, integrating the god of scorched earth, war, and pestilence into the domain of the dead.3