[The following is a retelling of the story of Goddess Durga’s defeat of Mahishasura as narrated by my grandparents. There may be subtle variations to the story, and I do not claim to be an authority on the subject]
Long, long ago, heaven was divided into two halves – the Devas, and the Asuras. Both sides were locked in a prolonged battle over who would rule heaven. The Asuras had an advantage – their leader, Mahishasura, had received a blessing that ensured that no man would defeat him. So naturally, the Devas did not win.
Devastated by their defeat, Indra, King of the Devas, sought help from the higher Gods, Brahma, Vishnu and Shiva. They sympathised with Indra, and projected their energy to create ten-armed warrior Goddess, Durga.
Since Durga was created in the form of a woman, Mahishasura’s blessing could no longer protect him. To further help Durga, each of the Devas gave her a weapon. Thus prepared, Durga went face Mahishasura, to save the Devas.
Asuras can take the form of any animal. Mahishasura took the form of a buffalo. After a gruelling fight, Durga killed the buffalo as well as Mahishasura who emerged from within it, thereby defeating the Asuras and granting victory to the Devas.
Today, Durga represents the destruction of evil, with Mahishasura being the personification of said evil. Every year, Durga Pujo celebrates the Goddess’s triumph over evil, and her return with her children to her motherland, India.