Mandodari requested Ravana at night to save the clan by going to Rama and seeking his benevolence. He is the incarnation of the God of the universe. He is all powerful, she said. Ravana went to the Mahakal (God of time) temple to pray for strength.
When Indra, king of Devas, saw Rama fighting on foot in the battlefield, he sent his chariot and charioteer to Shri Rama and requested Rama to ascend the chariot in order to quickly kill Ravana and help the Devas. To respect the sentiments of the Devas, Rama accepted the chariot.
Ravana now used various mayavi, mysterious and magical devises to distract Rama. But Rama rendered all his tricks useless. He reduced all the energy and power of Ravana and in the end, with the help of Brahmastra (weapon of Brahma), he killed Ravana. The Devas in heaven rejoiced and blew kettle drums and sounded drums to celebrate the end of the rakshasas who had terrorized them for so long. They showered flowers on earth. The gandharvas and apsaras (celestial beings), the celestial musicians and dancers, sang and danced in celebration.
Meanwhile Vibheeshan mourned the death of his brother and Mandodari cried and wept inconsolably.
Vibheeshan performed all the rites and rituals for the final ceremony of Ravana. Lakshmana went to the palace of Lanka for the coronation of Vibheeshan as the king of Lanka.
With the permission of Rama, Vibheeshan arranged for Hanuman and other warriors to escort Sita to the camp where Rama awaited. The vanaras were anxious to get a glimpse of Sita. Respecting their sentiments, Rama allowed all of them to gaze at Sita.
In the presence of all the people Rama asked Sita to prove her purity and with an ordeal through fire, Sita underwent on Agni pariksha (proof by fire). Sita proved her pure status.
Death of Ravana, Ramayana – The Epic Story