Legend has it that two demons were vanquished by The Goddess Durga and her other manifestation, Kali as described in the ancient Upanishad scriptures called the Devi Puranas. Durga Puja celebrates the triumph over one of the demons – Mahisashura; Kali Puja – victory over the other – Raktabija.
According to scripture, Mahisasura’s father, Rambha, a demon king, had fallen madly in love with Mahishi, an Asura who could transform into a buffalo at will, at times maintaining hybrid form. He took her as his wife – and while with child, she was abducted by an ex-lover from her abode. The two fought; Rambha was killed.
Devastated, a grieving Mahishi leapt on to Rambha’s funeral pyre – an act which moved Lord Yama, God of Death. He halted the migration of Rambha’s soul, and redirected it into Mahishi’s womb. Out of the cremation fire emerged twins: Mahishasura, the son conceived of Rambha and Mahishi; and Raktabija, the reincarnated Rambha.
Mahishasura was a great soldier and his penances garnered him a boon from Lord Brahma, the creator in the Hindu pantheon. Arrogance would be his undoing: he asked to be impervious from death by either man or god, for surely he had nothing to fear from a mere woman.
Raktabija had also gained a special power: wherever his blood would fall, his seed would sprout, and he would be duplicated.
The demon army led by these twins were terrorizing the known worlds. So, the Devas(Demi-Gods) appealed to the holy trinity Brahma, the Creator, Vishnu, the Preserver, and Shiva, the destroyer to eliminate the threat. Together, they created the ten armed Goddess, Durga. They, along with the Devas armed and empowered her. In turn, she manifested their combined power.
Unprotected from being killed by a female and underestimating the female he faced in battle, Mahishasura proposed marriage to Durga when he met her on the battlefield. He, being ego personified left her enraged at his arrogance. In killing him, Durga demonstrates that we must conquer our ego on the path to enlightenment.
Similarly, Raktabij represents desire for which there is no end to satisfaction – one drop of blood begetting another demon, one craving satiated giving rise to another one. Durga, assuming the form of Kali, cut the head off of Raktabij and drank all of his blood, so not even a single drop would fall: her solution being to eliminate all desire before it has a chance to sprout.