Mahishasura was a devil who can change shape quickly. He had a blessing that he will never be killed by a man. When the battle started between God who was led by Indra & Devils led by Mahishasura, Gods were defeated. Being disappointed, Gods combined their divine energies into the Goddess Durga. Maa Durga then fought with Mahishasura for 15 days and killed Mahishasura, the demon.