Skand Puran, the story available at website ………………………

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Skand Puran, the story available at website ………………………
EXECUTION OF SHUMBH, NISHUMBH & MAHISHASUR
Demons ( Asurs) by Hindu mythology:
1.) Mahishasur: the king of Purliya Distt.of West Bengal & Mysore (Karnatka) ( todays Yadav caste),
2.) Prachanda: the king of upper Himalya (Nepal) ( today Maoist),
3.) Chamar: The king of Asur (Shiva) North India ( Today Dalit),
4.) Mahahanu:शिव। (. तक्षक जाति का एक प्रकार का साँप), Shiva's people/follower,
5.) Dahan: The demon of Sambalpur distt. Haryana( Today's Jat ),
6.) Vikataksha: Chief of the Asuras, in the reign of king Bali.Non-Aryans, (Today non-Brahamins)
7.) Mahamauli: Barbarian ( outsider of the Aryavarta),Non-Aryans Today Non-Brahamins
8.) Shumbh and Nishumbh: The kings of Vindyachal mountain in Madhya Predesh ( todays Dalit & backwords in Vindhyachal distt) 
The demons named Shumbh and Nishumbh had received boons from Brahma according to which no deity, demon or Man could kill him. Shumbh and Nishumbh became excessively arrogant and started tormenting the deities. All the deities including Lord Vishnu went to Lord Shiva and requested for his help. Lord Shiva assured them that both the demons would be killed at the opportune time. The deities were satisfied and returned back to their respective abodes. Parvati was of dark complexion. She thought that Shiva would be more affectionate towards her if she somehow discarded her dark skin. She eventually discarded her dark skin at a place and it instantly got transformed into 'Kali Kaushiki'. She then did an austere penance at Vindhyachal mountain. At that time, Shumbh and Nishumbh lived there. When both the demons saw goddess Kali Kaushiki her divine beauty infatuated them. But Goddess Kali Kaushiki ultimately killed both of them.
Mahishasur--the demon sent a female messenger to convince Parvati into marrying him. The female messenger disguised herself as a female hermit and tried to impress all the three goddess who were keeping surveillance by praising the glory of Mahishasur--
Goddess Chamunda (Durga) severed the heads of 'Chand' and 'Mund' with her chakra. Mahishasur was enraged and he attacked goddess Durga. Some other demons like Prachanda, Chamar, Mahamauli, Mahahanu, Ugravaktra, Vikataksha and Dahan also came forward to help him but each one of them was killed by goddess Durga. Now, Mahishasur's anger crossed all limits and he menacingly ran towards goddess Durga. A severe battle was fought between both of them. When Mahishasur realized that the goddess was dominating the battle, he started changing his guises frequently. He tried to dodge goddess Durga by transforming his appearance into that of a boar. But, goddess Durga kept on chasing him. Then, Mahishasur became a lion. This way he kept on changing his appearances frequently to avoid getting killed by goddess Durga. Once he transformed himself into a buffalo but goddess Durga attacked him.
This way, Mahishasur was forced to change his appearance frequently on account of relentless attack by goddess Durga. Ultimately Durga killed him. Goddess Durga picked up his severed head and danced in joy. The deities were relieved at the death of Mahishasur.
Ref.:- http://www.bharatadesam.com/spiritual/skanda_purana.php