Once upon a time there lived a demon (Asura) named Mahisha. He found great happiness in hurting people. Once, he decided to pray to lord Brahma, who he thought would give him a boon, which would make him invincible.
Mahisha performed severe penances praying and fasting for months as he stood on one foot. The three worlds trembled under the strength of his penances and a pleased lord Brahma came to give him a boon. Mahisha asked for immortality, which the lord said he could not have as every creature that was born had to die. Mahisha decided that he would ask for a boon that would make him as good as immortal. He asked that no man should be able to kill and, if he had to die it should be only at the hands of only a woman. He was sure that no woman could ever fight against him however strong she may be.
Now that Mahisha was invincible, he and the other asuras went about hurting and killing everyone on earth and then went on to the heavens to fight the Gods. Even Indra's thunderbolt could not withstand the asura attack. Mahisha drove out the Gods and took over Indra's throne. Mahishasura started harassing all pious people who continued praying to Vishnu or Shiva. The Gods and people were depressed and decided to ask lord Shiva for help. Lord Shiva, Vishnu, and Brahma concentrated hard and used their radiant energies, which were joined by the energies from Indra, and the other Gods. This godly energy took the form of a divine lady with thousand arms. In each arm she carried a weapon belonging to all Gods. This was Durga Devi.
She mounted a fiery lion and roared. It was a roar that shook mountains and created huge waves in the seas. Even Mahisha was worried for a second, but his vanity took over when he saw that the terrible form was that of a woman. Durga created a large army from her breath to fight Mahisha's army and then fought with Mahisha who came in the form of a Buffalo. As he struggled to set himself from the Buffalo form she killed him with her sword delivering the earth and heavens of the Burden called Mahishasura.
In Mysore (Karnataka) atop the Chamundi hills, you can see a big statue of Mahishasura. Here there is a temple dedicated to Durga also known as Mahishasura mardini or Chamundi.