Ravana was the king of Lanka. In Indian folklore he abducted Sita the wife of Lord Rama and took her to Lanka (Ceylon). Rama then mounted an invasion army that crossed to Sri Lanka. In an epic battle Ravana was defeated and killed and the Lord Rama brought Sita back. This is all related in the Ramayana. He is reported to have ruled during the period 2554-2517 BC. Ravana was supposed to have 10 heads, but Lankans feel that this represented the 10 kingdoms that he ruled. Ravana is very much a historical figure and historians generally credit him as the ruler of Lanka with his headquarters at Polonnaruwa which is located in North Central Sri Lanka.
There has been a revival of sorts in Lanka and it is pointed out that Ravana had made great advances and also invented the pushpaka vimana or the airplane. In addition he is held in high regard as a physician. 7 medical books are credited to Ravana which were originally written in Sinhala and later translated to Sanskrit. Ravana’s medical books are NadiPariksha, Arka Prakashata, Uddisa Chiktsaya, Oddiya Chikitsa, Kumara Tantraya and Vatina Prakaranaya.
Hindu texts of that period also credit Ravana as a man who ruled the 3 worlds and was a follower of Shiva, the creator and destroyer. He is reported to have been administered the juice of immortality by Shiva. He had also mastered the Vedas and all the religious books of the Hindus.
Hindu texts also agree that Ravana was a virile king, who was fond of women, his favorite wife being Mandodri. Indian texts in particular the Ramayana mention that Ravana abducted the lovely Sita from the Jungles of central India and took her to Lanka.
Why did Ravana abduct Sita? The Ramayana mentions he did this as an act of revenge for his sister Supernlakha, whose nose had been chopped off by the brother of Rama, Laxman. When Ravana took Sita to Lanka he kept her with him in a separate palace in Central Lanka. He visited her everyday and lavished gifts and presents on Sita. The Ramayana mentions that during this entire period of captivity of Sita, she remained chaste. This does not gel with the actual facts as a beautiful woman in the custody of a powerful king cannot be left untouched.
Hindu texts do not discuss the relationship of Ravana and Sita in any detail. He did lavish gifts on her but the core issue is untouched. As Rama was the 9th avatar of Vishnu perhaps in deference to Rrama this issue is left untouched. But now people in the 21st century can draw their own conjectures.