Once there was a great rishi(saint) named Pulastya. He had a son named Vishrava. He married the daughter of Rishi Bhardwaj and gave birth to Kuber(god of wealth). He led an austere life and pleased Lord Brahma through his penance due to which he blessed him with the golden city of Lanka.

Later on Somali Rakshas(demon) told his daughter Kaikayi to marry Vishrava. One day she went to the cottage of Rishi Vishrava at the time of dusk. The rishi told her that since she came at dusk, she would give birth to Rakshasas(demons). She accepted his terms and later on gave birth to Ravana and Kumbhkarna.

One day, Rishi Vishrava was talking with his wife and she told him that she would have given birth to 10 sons had he not been meditating for so long. He told her that to compensate for her loss; he would give her such a son who would have 10 heads and 20 hands. Thus Ravana was born.