The story of the Birth of Vishrava, father of Ravana, is found in the Uttara Kanda of Ramayana. In the Krita Yuga, Brahma had a son named Pulastya. To observe tapas and other austerities he resided in an ashram of Sage Trinavindu.
But daughters and sisters of devas, sages, serpents, and damsels went into the ashram and sported. Their activities disturbed Sage Pulastya.
When their activities became unbearable he warned them that anyone who comes within his range of vision would become pregnant.
But once the daughter of Sage Trinavindu who had not heard about the curse roamed in the hermitage of Sage Pulastya and went near him.
Soon she turned pale and signs of pregnancy became visible.
Finding out her state, she went and stood before her father.
Sage Trinavindu soon realized the reason for her daughter’s sorry state.
He took his daughter to Sage Pulastya and asked her to accept her.
The maiden resided in the ashram and took care of her husband.
Pleased with her, Sage Pulastya blessed her with a son who will have all the qualities of him.
As the maiden had heard Sage Pulastya reciting the Vedas when she was pregnant he will be known as Vishrava.