Mahishasura is a combination of mahisha or a buffalo and asura or demon. Mahishasura was partly-immortal; no man could kill him. Mahishasura beat the god in a battle between Asuras and Devas, when the gods were led by Indra. Dejected, the Devas then combined divine energies bring goddess Durga to life. Riding a lion she led the battle and killed the deamon. This form of Durga is known as Mahishasuramardini (the killer or Mahishaura).
According to Christopher Fuller, Mahishasura symbolically represents forces of ignorance and chaos hidden by outer appearances.