Narzuro was once a powerful Black dragon but later he was made a devil after gaining a sufficient number of worshippers, and earning the title "devil of Destruction".
Narzuro was the cause of the utter and total Destruction of the great kingdom Andaro, as well as it's transformation into the barren wasteland it is today. This granted him the title of "Destroyer".
As this became more known Narzuro started gaining Worshippers, most of these worshippers were people who wanted the Destruction of something or someone.
When enough worshippers were gained he became a Devil with "Destruction" as his portfolio.