The demon started off as the ruler of Pragjyotishapura (in Assam), but soon conquered the entire world. He attacked the gods and drove them out of heaven. The gods went to Shiva, but Shiva knew Vetrasura to be invincible. Shiva could certainly not kill him. Shiva therefore led the gods to Brahma.

Brahma was, at the time, seated on the banks of the holy river Ganga and chanting the gayatri mantra (a sacred incantation). He was oblivious of all that went on around him and paid no attention to Shiva and the other gods.

But a goddess named Gayatri suddenly appeared. (She seems to have been a personification of the gayatri mantra.) The goddess had eight arms and was armed with all sorts of weapons. She was clad in a white sari. Gayatri devi fought with the demons for one thousand divine years and killed Vetrasura.

Shiva sung her praises. When Brahma’s meditation was over, he earmarked a place in the Himalayas for the goddess to live in.
Since Vetrasura was killed on navami tithi, navami is set aside for the worship of the goddess Gayatri.