According to a narrative in the Devi Mahatmya, Durga was created as a warrior goddess to fight an asura named Mahishasura. He had unleashed a reign of terror on earth, heaven and the nether worlds, and he could not be defeated by any man or god, anywhere. The gods went to Brahma, who had given Mahishasura the power not to be defeated by a man, but Brahma could do nothing. The gods then went to Vaikuntha-the place where Vishnu lay on Ananta Naag. They found both Vishnu and Shiva, and Brahma related the reign of terror Mahishasur had unleashed on the three worlds. Hearing this Vishnu, Shiva and all of the gods became very angry and beams of fierce light emerged from their bodies. The blinding sea of light met at the ashram of a priest named Katyayan and Durga emerged from this pool of light. The goddess Durga took the name Katyaayani from the priest and emerged from the sea of light. She introduced herself in the language of the Rig-Veda, saying she was the form of the supreme Brahman who had created all the gods. Now she had come to fight the demon to save the gods.
Upon encountering Durga, Mahishasura underestimated her, thinking: "How can a woman kill me, Mahishasur-the one who has defeated the trinity of gods?" However, Durga roared with laughter, which caused an earthquake and made Mahishasur aware of her powers.
Mahishasura rampaged against Durga, changing forms many times. First he was a buffalo demon, and she defeated him with her sword. Then he changed forms and became an elephant that tied up the goddess's lion and began to pull it towards him. The goddess cut off his trunk with her sword. The demon Mahishasur continued his terrorizing, taking the form of a lion, and then the form of a man, but both of them were gracefully slain by Durga.
Then Mahishasur began attacking once more, starting to take the form of a buffalo again. The patient goddess became very angry and threatened the demon. When Mahishasur had half emerged into his buffalo form, he was paralyzed by the extreme light emitting from the goddess's body. The goddess then resounded with laughter before cutting Mahishasur's head down with her sword. Thus, Durga is also known as Mahishasurmardhini-the slayer of Mahishasur.
Interesting facts about Durga
It is found in the Markandeya Puran that Devi Durga was worshipped for the first time by king Suratha who was born in the Kshatriya clan during the period of second Manu.
There are many incarnations of Durga: Kali, Bhagvati, Bhavani, Ambika, Lalita, Gauri, Kandalini, Java, Rajeswari, et al.
Harshananda, S. (1982). Hindu Gods and Goddesses. Chennai: Sri Ramakrishna Math.
Wilkins, W. (1882). Hindu Mythology. New Delhi: Rupa. co.