Trikuta wanted to marry Prince Rama of Ayodhya. But Rama said, "I already have a wife and will not take another."

So Trikuta went to the mountains to live as a hermit. But there Bhairava, a sorcerer, forced his lustful attentions on her. Disgusted by his behaviour, she left her hermitage and sought refuge in a cave. He followed her there and refused to leave her alone.

Finally, after being chased across many hills and valleys, Trikuta decided to run no more. She turned on her tormentor with a sword and after a great battle, succeeded in beheading him.

The beheaded Bhairava apologised to the goddess and accepted her as his mother. From a lustful man he was transformed into an innocent child, by the grace of Trikuta.

Thereafter, the hill-dwellers began revering her as a goddess. She came to be known as Vaishnavi, as it is said that she will be Vishnu's bride when Kali Yuga, the dark age of spiritual blindness, comes to an end.

Source: Devi The Mother-Goddess: An Introduction By Devdutt Pattanaik.