Introduction
Mura is an Asura, the general or guardian of Narakasura's city of Pragjyotisha. He is remembered chiefly for his death at Krishna's hands, from which Krishna takes the name Murari, "enemy of Mura".
The fight
According to the Bhagavata Purana, Mura had seven mouths and guarded the approach to Narakasura's stronghold, which was further protected by moats, fire, water and wind. Krishna cut off Mura's heads with his discus as he fought his way to Narakasura.
The name Murari
The epithet Murari, used for Krishna in many devotional texts and hymns, comes directly from this episode, and is one of his more common names in later Vaishnava poetry.