Story of how Durga became MahiShasura-Mardini
Ahishasura's father Rambha was king of the Asuras(Demon), and he once fell in love with a Water Buffalo(Princess Shyamala, cursed to be a buffalo)Mahishasura was born out of this union. He is therefore able to change between human and buffalo form at will (mahisha is Sanskrit word for buffalo).
Mahishasura got boon from Brahma and was very powerful, and became invincible. He started terrorising people, and occupied heaven and drove all Devas.When devas learnt he is Invincible and only Devi Jaganmatha can defeat him, they all joined their energies into shakti. A very powerful band of lightning emerged from the mouths of Brahma, vishnu and shiva and a young, beautiful female virgin with ten hands appeared. All the Gods gave their special weapons to her. This Shakti coalesced to form the goddess Durga. Riding on a lion, who assisted her.Durga fought Mahishasura. The battle raged for nine days and nights. Finally on the tenth day of Ashvin shukla paksha, Mahishasura was defeated and killed by Durga.Durga is therefore called Mahishasura-Mardini (literally the slayer of the buffalo demon), the destroyer of Mahishasura.