Mahishasura

In Hindu mythology, Mahishasura was an asura (demon).

When enraged, he took the form of a green buffalo.

Pleased with his meditation, Brahma gave him a boon that no man or god will be able to kill him. After getting the boon, he started terrorising Swarga Loka (Heaven) and Prithvi Loka (Earth). He invaded Swarga Loka and defeated the king of gods Indra and took control of Swarga Loka. He drove all the devas (gods) out of heaven. The gods went to the trio of Brahma, Vishnu and Mahesh to seek their help. They were enraged on hearing about the invasion of heaven. Their anger took the form of a young woman. It is said that Shakti – an incarnation of godessess parvathi, combined with the powers of Lakshmi and Saraswathi was given powers by all the devas to fight mahishasura. The godessess then attacked Mahishasura's evil empire with the gods. After nine days of fighting, Durga was able to slay the evil demon.

The festival of Durga Puja is celebrated to celebrate this victory of good over evil.