Shumbha and Nishumbha were the two Asura (evil forces) brothers who managed to obtain a boon from lord Brahma that they will not be killed by any male whether human being or animal / bird etc. but by a woman only. They desired this boon thinking that they will live for ever as they did not know of a lady who was so powerful as to kill them both in a war. Thus empowered, these demons started disturbing the prayers of the saints torturing them and asking them to worship the duo rather than any other deity. The Devtas realising that they cannot defeat the two brothers recapitulated the promise of the Goddess to come to their help whenever asked for. Gods then together praised the deity by saying that she was omnipotent, resided in each and every emotion and sensation of all beings in this universe.
The goddess Durga, riding a lion appeared and asked Indra, 'Lord of gods, I am pleased. What brings you here?
Indra paid obeisance to the goddess and said, "Mother, my throne has been taken away by two demons named Shumbh and Nishumbh. You are all powerful, omniscient and omnipresent. Nothing is hidden from you. You can protect us from those demons."
Durga said, "Indra! one who forgets the power of supreme and considers himself to be almighty driven by arrogance, drowns himself in drinking, singing, dancinig and debauchery taken over by the evil desires, he falls down like you have. That's the cause of your downfall. I can't help you."
"Don't say that, mother," Indra begged. " A big mistake I have committed. I seek your forgiveness. I will never go bad again. Give me a chance to redeem myself. We have learned the lesson. Get me and my subjects out of this trouble."
Seeing Indra repentant, the goddess, Durga took pity on him and spoke, "Indra! At this time all my powers are vested in the daughter of the Himalaya, Parvati. And She is in meditation of Lord Shiva. Go to Her and pray. Only She can suggest a way out for you." The goddess vanished.
The Gods went to the place where Goddess Paravati, the daughter of the king of the mountains was in deep meditation. The gods prayed. The prayers of the Gods fell into Her ears, she was moved to pity and thus she created another Goddess from her own being who came to be known as "MahaGauri". After the emergence of the Goddess, Parvati turned black thus giving the name 'Kalika' also to the goddess. The Goddess thus born was supremely beautiful and when Chanda and Munda, the disciples of the demon brothers Shumbha and Nishumbha set eyes upon her they at once reported of her beauty to their masters. Upon hearing of her beauty, the two brothers sent a marriage proposal to the goddess. The Goddess told them that she has taken a vow to marry the one who can vanquish her in battle and take her forcibly. Thus incited into a battle by the Goddess, Shumbha and Nishumbha first send a large army and their able Generals to beat this beautiful lady, but on being defeated, they sent Chanda and Munda whom also the Goddess defeated assuming the form of Maha Kali and thus the goddess was also named 'Chamunda', the one who beheaded the Asuras Chanda And Munda.
As the battle became more and more fierce, seven more Goddesses or Shaktis emerged from the bodies of seven Devtas to assist the Goddess in this battle. At that time was sent to the battlefield, the all powerful demon Raktabija who had been granted the strange boon that wherever his blood fell on the ground it would take the form of a 'Bija' or seed and from it will sprout another Raktabija. To slay him the Goddess asked Kali to lick the blood of the Raktabija before it fell on the ground. The goddess in the meantime beheaded the demons. All the Shaktis then together killed Nishumbha and his army leaving just Shumbha. When Shumbha realised that he has been outnumbered, he taunted the Goddess that she alone cannot defeat him and needed the help of seven other Goddesses to do so. The Goddess got infuriated and merged all the Shaktis into herself and single handedly killed Shumbha thus freeing the world of evil.