Many a times Devtaa called upon Durgaa to kill Asur whom they could not overpower them themselves. Durgaa killed 9 of them assuming 9 forms. These 9 forms are worshipped during Nava Durgaa period of 9 days which comes two times in a year. One of the most famous Asur among them killed by her was Mahishaasur.

Mahishaasur became very powerful after doing Tap for Brahmaa Jee. Brahmaa Jee bestowed him with the Var that "no Devtaa or human being will be able to kill you." After getting this Var, he first won Prithvi, then came to win Devtaa. He captured Indra Dev and started ruling Indra Lok. It was too much for Devtaa to continue like this, so other Devtaa went to Brahmaa Jee to save the Universe from Mahishaasur. Brahmaa Jee went to Vishnu Bhagavaan to ask for His help. Vishnu got very angry to hear this and out of His anger came out a Power Ball. All Devtaa gave their share of powers to this Power Ball and that Power ball assumed the form of a woman named Durgaa. She was so powerful, she was so  beautiful. Many Asur came to see her and wished to marry her.

She told that she would marry him only who will win her in war, so many Raakshas came to win her but lost their lives. Mahishaasur also came to win her. The battle between Durgaa and Mahishaasur continued for 10 days and then he was killed by her. That is how she was called Mahish Mardinee.

Durgaa Poojaa coincides with Raam's story. Raam's fight with Raavan also continued for 10 days and then Raavan was killed by Raam on the 10th day. Whether the two are inter-related or not, is not clear.

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