According to Gunnabhadra‘s Uttara Purana, Ravana disturbed the asceticism of Manivati who was daughter of Amitavega of Alkapuri which made her furious and pledged to take her revenge from him.
Years later Manivati is reborn as Daughter of Ravana and Mandodari, but according to prediction of astrologers his eldest child would destroy his lineage hence he ordered an assassin to assassinate the infant. The assassin obliged to do so but when he saw the baby’s face he could not gain the courage to kill someone so pure, so he put her into a casket and buried in the soil, leaving the rest to fate.
The place where he buried the infant was in the Kingdom of King Janaka, where an untimely drought occurred. After loads of Yagya (devotional offering) The goddess of fertility asked King Janaka to plough the land in order to bring back the it’s fertility. King Janaka started ploughing after sometime his furrow struck a casket in which a beautiful baby girl was found. King Janaka and his wife Mithila were childless hence they adopted her and named her Sita- furrow, she is also known as Janaki -daughter of janak.
Ravana was relieved that his daughter had survived and at the same time overjoyed that she was still a princess. Ravana’s happiness quadrupled as he heard that his daughter was now married to Ram of Ayodhya (an incarnation of Lord Vishnu). All was fine until Ram was sent on exile to the forests for 14 years.
Ravana could not bear the thought of Sita suffering in the forests. He endeavored to get Sita back to Lanka and put an end to her misery. People of Lanka thought it was Ravana’s way of claiming vengeance on Ram and his brother Lakshman, for having cut off the nose of his sister Surpanakha. Even Mandodari, his wife did not understand him. She remembered him murmuring the name ‘Sita’ in his dreams and thought Sita was the love of his life. Ram declared war on Lanka. Vibhishan, Ravana’s younger brother unaware of his intentions, joined the army of Ram. Ravana sent his sons and brothers into the war but all were killed.
Ravana had his reasons for not waving the white flag and accepting defeat:-
Firstly, it was his immense love and affection towards Sita. He just could not send her back to the forest to suffer.
Secondly, he knew Ram was an incarnation of Lord Vishnu himself and death at his hands would give him salvation. He fought his last battle valiantly and lost his life to Ram. Ravana did receive his salvation, but in the process Lanka was destroyed. The words of the priest did come true. Sita did bring destruction to Lanka.
There’s another story according to Hindu mythology which depicts why Ravana was not able to touch Sita ever in his life which follows as below:
Rambha & Ravana’s story
Rambha was the queen of the apsaras. She was one of the most beautiful women ever created. She was made by the waves of the ocean, by the tides of the sea. She was made out of love between the sand and the life source of all living things: water.
Ravana was intrigued with Rambha’s beauty and one day as she was going to see Nalakubara, Ravana came around a bush and tried to seduce her. When Rambha told Ravana that she was married he became infuriated and raped her. Luckily, Nalakubara did not blame Rambha and loved her anyways. Nalakubara cursed Ravana that he ,with lustful feelings cannot touch a woman without her consent or he will die. It is because of the curse only that he could not touch sita.