The demon no one could beat
Long ago there lived a mighty demon called Mahishasura. He could change his shape whenever he liked, but his favourite form was a giant black buffalo.
He prayed for years until Brahma, the creator, gave him a blessing: no man and no god would ever be able to defeat him. He was so proud that he never thought a woman could beat him.
“Then I can never lose!” Mahishasura laughed. He attacked heaven, and after a war that lasted a hundred years he chased the gods out and sat on Indra’s throne himself.
A goddess made of light
The gods went to Brahma, Vishnu and Shiva and told them everything. The three great gods grew so angry that a blazing light burst out of them.
Light poured out of every other god too. All of it joined into one great glowing mountain, brighter than a thousand suns.
And out of that light stepped a goddess with ten arms. She was Durga. Mahishasura’s blessing kept him safe from men and gods. It could not keep him safe from her.
The laugh that shook the worlds
Durga raised her head and laughed. Her laugh was so loud that the mountains shook, the oceans rose in giant waves and the whole sky rang.
Mahishasura heard it in heaven. “Who dares?” he roared, and he sent his whole army against her.
Durga’s lion leapt into battle. Her ten arms moved like lightning, and one by one Mahishasura’s greatest generals fell.
Nine nights of battle
Then Mahishasura came himself. He charged as a giant buffalo, tossing mountains with his horns and whipping up the oceans with his tail.
Durga threw her noose around him. In a flash he became a roaring lion. She struck, and he became a warrior with a sword. She struck again, and he became a huge elephant that grabbed her lion with its trunk!
Durga cut the trunk away, and the demon turned back into the great buffalo, bellowing so loudly that all three worlds shook. Night after night the battle raged.
The ninth night
On the ninth night, Durga laughed once more. “Roar while you can, foolish demon,” she said. “Soon the gods will be cheering!”
She leapt onto the buffalo and pinned it down with her foot. As Mahishasura tried to escape from the buffalo’s body in his true form, Durga struck with her blazing trident.
With one mighty blow, the demon who thought no one could beat him was defeated.
The day of victory
The gods rained flowers from the sky. Music filled heaven, and the whole world cheered.
That is why the nine nights of Navratri end with Vijayadashami, the tenth day, the day of victory.
In Mysuru she is worshipped as Chamundeshwari, and the city celebrates with a grand royal elephant parade called the Jamboo Savari. In Bengal, families celebrate Durga Puja for days and say goodbye to her on Vijayadashami.
Evil may be strong and proud, but when good stands together, it always wins.
Source: Devi Mahatmya (Markandeya Purana), chapters 2 and 3; Mysuru Dasara and Bengal Durga Puja traditions. The nine nights of battle come from the Navratri tradition.