The description of Durga given by Sage Vyasa in his 'Markandeya Purana' is available in the form of 'Durga Saptashati'. Durga is the symbol of the force which is the reflection of Power Supreme. Durga inspires humans into labour, work, endeavour, adventure and industry. The worship of this force give birth to 'Shakti' sect in Hinduism. Durga concepts springs from the knowledge of Vedas and Upanishads. In a nutshell, for the destruction of evil forces and non-believers, God's power took birth in the form of Durga.
In an ancient age, Rambhasura was born in a demon dynasty. His son was called Maharshi. The son was as powerful as a wild bull. He was a sorcerer as well. Maharshi could become any beast. Bull buffalo was his favourite form. So, the demons preferred to call him Mahishasur, the bullish demon. He was arrogant and debauch but a favourite devotee of Brahma.
Once he did penance of Brahma standing on one leg only for years. He chanted 'Om Brahma devaye namah'. Lord Brahma appeared to him and asked him to wish for a boon.
Mahishasur asked for immortality.
"No, I can't give you that," Brahma declined and added, "Whoever is born must die. Ask for something else."
Mahishasur said after some thought, "Lord, then bless me that no man or beast kills me. Death at the hands of a woman won't matter." The demon thought that the soft and weak female won't be able to kill him anyway. Brahma granted the boon."
When Mahishasur got his wish granted, he began tormenting humans. He and his demon army would be raid the Yajnas of sages and kill them. The abduction of women and dishonouring them became the order of the day. The reign of terror made the humans cry for some divine help. One day, driven by megalomania, the demon raided god's home Alkapuri, the capital of Lord Indra, the king of heavens.
A battle between the demons and gods began and raged on. The rivers of blood started flowing. The gods were caught on the back foot and couldn't face the onslaught of Mahishasur. They fled.
After the retreat of the gods, Mahishasur captured the throne of Indra. He declared himself to be the new Indra, the lord of heavens. Announcement was made in all the three worlds for everyone to worship Mahishasur as god supreme. Anyone not doing so was to be publicly executed.
The announcement shook the three worlds. The new lord ordered the heavenly beauties of Alkapuri to dance for demons. When they refused Mahishasur terrorised them. The gods ran to the triology of Brahma, Vishnu and Mahesha. The triology asked the gods to wait for the favourable situation.
The woes of gods pained Brahma, Vishnu and Mahesha. They must do something. The three invoked their powers. The powers in energy forms flashed out of their bodies and came together to become an intense light ball which took shape of the goddess Bhagawati Durga riding a lion. Lord Vishnu gave Her His chakra, Shiva handed Her His trident, Brahma gave lightning, Indra gave Vajra, the thunderbolt, Sun armed Her with nuclear weapon, Moon handed Her sword, Wind transferred his speed to Her and other gods gave their respective powers to Durga.
Thus, armed with incredible weapons, the ten-handed goddess, Durga stood there astride the lion while the gods prayed to Her to rid them of the terror of mighty demon, Mahishasur.
Durga screamed in anger. "Don't grieve, gods. I shall slay that demon. Meanwhile, you take shelter in these mountain ranges and wait." Then, she vanished.