Mahishasura, the buffalo demon, was killed by Goddess Durga after he had defeated the gods and taken over the heavens. Durga was created by the combined energies of all the gods (Trimurti - Brahma, Vishnu, Shiva) to defeat the demon who could not be killed by any male deity. She fought him for nine days and nights (Navratri) before slaying him. While Kali is a fierce form of Durga, it was Durga who specifically killed Mahishasura, earning the title 'Mahishasuramardini'.