While it doesn't involve a demon-king, it does feature Gilgamesh, a powerful king who embarks on a dangerous quest to find the secret to immortality after the death of his beloved friend Enkidu. His journey takes him to the underworld, where he confronts the goddess Ishtar, who can be considered a powerful, albeit not demonic, figure.
The poem explores themes of loss, mortality, friendship, and the search for meaning in life. It's a classic example of an epic poem with its grand scope, mythical elements, and powerful imagery.