According to Hindu mythology, Mahisashura, the buffalo demon, had earned the favor of Lord Shiva after long meditation and prayers. Shiva, pleased with the devotion of the demon, blessed him with a boon that he could not be defeated by any God or man. Empowered with the boon, Mahishasura started killing people mercilessly and even drove the Gods out from heaven. With his reckless marauding, this demon posed a constant threat to the existence of the universe.
Indra, the King of gods, and Kumara, Commander of the celestial armies could not stop Mahisashura. In despair, the Gods called on Vishnu to deliver them from this demon. Vishnu confronted Mahisashura first as Narasimha the man lion and then as Varaha, the boar, but each time he faced defeat. Shiva, the supreme ascetic, was disturbed by the violence and glared at Mahisashura with his third eye, unleashing the fire of doom. However, even the power of Shiva's third eye - capable of destroying the three worlds - could not arrest Mahisashura's march.
At this time, a stream of lightning dazzled forth from the mouth of Brahma, in the form of the goddess Brahmi. She rode a swan and held books of wisdom in her hands. Simultaneously, the essence of the other Gods emerged taking female forms. From Indra arose Indrani, bearing a thunderbolt and riding an elephant. From Kumara rose Kumari holding a lance and riding a peacock. From Vishnu rose Vaishanavi on an eagle with a disc whirling on her finger. From Varaha came the sharp-tusked sow Varahi. From Shiva arose Shivani, riding a bull and bearing a trident. These Goddesses rose to the sky and merged with each other in a flash of blinding light. The sounds of conchs shells, drums and bells filled the air. With bated breath, the Gods watched the light. From this heavenly light emerged a beautiful Goddess, Durga - the inaccessible one. Then all the Gods bestowed their special weapons upon Durga. Shiva gave his trident, Vishnu his disc and mace, Indra his thunderbolt, Kurmara his lance, Brahma his bow.
Thus armed, Durga mounted a lion and prepared for battle. A great battle commenced. Mountains shook, oceans trembled, clouds scattered across the sky, as Mahisashura demon attacked Durga. He rushed her using all his wiles, at one time taking the form of a buffalo, sometimes as a lion, sometimes as an elephant. The Goddess broke the buffalo's horns with her mace, sheared the lion's mane with her lance, and cut off the elephant's trunk with her sword. She hurled weapon after weapon at the buffalo demon, but each time he managed to rise undefeated. Realizing that her heavenly weapons had no effect at all on Mahisashura, Durga threw them aside, dismounted from her lion and sprang upon Mahisashura's back. With her soft feet she kicked his head. The demon, immune to the weapons of all the Gods, fell senseless at the touch of Durga's feet. Durga then raised her trident and plunged it into the buffalo demon’s heart - conquering the unconquerable.
(Photo courtesy: www.agastyacodemahadevar.org/photo.php?id=17)
Once a year, in the autumnal month of Ashwin, Durga - the Goddess of Deliverance - comes home to her parents, together with her four children, Ganesha (the God of wealth), Laxmi (the Goddess of prosperity), Kartik (the God of beauty) and Saraswati (the Goddess of knowledge). Durga comes to earth on the seventh day after the autumn new moon. She is welcomed and honored as a family would welcome its married daughter, and love and affection bestowed upon her. Unfortunately, this visit lasts only three days, and on the fourth day Durga starts on her journey back to her husband Shiva's abode in the mountain kingdom of Kailash.
The occasion of Durga Puja commences with Mahalaya - the day when the fierce battle between the Goddess and demon Mahisashura ensued. The rituals consist of reciting shlokas (couplets) from Chandi (also known as Chandi Path) and evoking the powers of the Goddess Durga. The Durga idol is depicted as a resplendent ten-armed golden figure standing on a lion's back, each of her ten arms bearing a particular weapon, as she triumphs over the demon Mahisashura.