Mahishasura’s father Rambha was king of the asuras, and he once fell in love with Princess Mahishi, who was cursed to be a water buffalo; Mahishasura was born out of this union. He is, therefore, able to change between human and buffalo form at will (mahisha is Sanskrit word for buffalo).
According to Hindu mythology, Lord Shiva, pleased with hard penance and prolonged meditation done by demon king Mahishasura and granted the boon of immortality to him. Shiva declared that no man, demon, beast or deity would ever be able to kill him. However, Shiva played a trick here and he did not mention the women to the list.
After receiving the boon of immortality from Shiva, Mahishasura started terrifying people in three lokas – Heaven, Hell and Earth. Slaying and killing brutaly, he and his army of demons soon conquered the earth, and set their eyes upon Devaloka (Heaven) – the abode of the Devas (gods). Mahishasura defeated Indra, the King of Gods and drove all the gods out of heaven.
To overcome their misery, Devas came to the trinity of Gods – Brahma, Vishnu and Shiva for seeking help. Mahishasura’s violence enraged the trinity to a great extent. Brahma, Vishnu, Shiva and other gods united their powers and brought down a magnificient goddess named Shakti (Power).
The trinity of goddesses Laxmi, Parvati, Saraswati are believed to be shakti of Vishnu, Shiva and Brahma respectvely. Hence goddess Durga is said to be the merged form of Laxmi, Parvati and Saraswati. The name Durga was given to her after she killed a demon named Durg.
Mahishasura, when he saw the divine beauty of Goddess Durga, got mesmerized. Enchanted by her beauty, Mahishasura proposed to marry her. Durga agreed, but on one condition that Mahishasura would have to defeat her in battle. Egoistic Mahishasura did not consider Durga a threat and agreed immediately!
The battle continued for 9 days and nights. Mahishasura took different forms to kill Durga, but each of his attempts failed. At the end of the ninth night, Goddess Durga slayed Mahishasura.
On the nine days of Navratri, nine forms of Shakti are worshipped. Tenth day is called as Vijayadashmi = Vijay + Dashmi = Victory on tenth day. The tenth day that brought the triumph of good over evil.