An army of demons was gathered, and its king was the Buffalo,the green skinned demon Mahishasura. His vast army, with its weapons of iron, and its phalanxes of elephants and charioteers marched
on the king of gods, Purandara, defeating him. Then, Mahishasura usurped the throne of heaven. The victory was complete, and all the gods were driven from heaven. They went to the three great gods, Brahma, Vishnu, and Shiva. 

"We have been dispossessed from heaven, from our home!" they said, "how can it be that heaven is
filled with malevolent demons. How can such a place even be called heaven?" 
On hearing the gods' petition for assistance, the face of Lord Vishnu flushed with anger and a
brilliant light flooded forth. Soon, the faces of Lord Shiva and Lord Brahma burned with the same dazzling luminescence. The gods watched in wonder as great flames issued forth in all 
directions. The fires illuminated all the three worlds: the heaven, the earth, and the nether-world in penetrating light. (source: www.parallel.park.org/India/Durga)