Story of Viradha is found in the Aranya Kanda of the Ramayana. It happens in the Dhandaka forest during the exile period of Bhagvan Sri Ram. Viradha was a huge Rakshasa. He had got the boon that he will not be killed by any weapon. He wore a tiger skin and used to roam around carrying a trident with animals and humans impaled on its prongs.
Viradha makes a sudden appearance before Sri Ram, Mata Sita and Lakshman. Before the brother could grasp what was happening, the Rakshasa snatches Mata Sita.
Viradha was the son of Jaya and Shatahrada and he had got the boon from Brahma that he will not be killed by any weapon.
This boon made him arrogant and fearless. He roamed around the world attacking all living beings. He finally made his home in the Dhandaka forest.
Soon a fight ensued between Sri Ram, Lakshman and Viradha.
Sri Ram broke the trident of the demon. In a fit of rage, Viradha carried Sri Ram, Mata Sita and Lakshman deep into the forest.
The brothers let the Rakshasa have his way as they were being carried away in the same path they intended to travel.
Finally, when they reached the dense forest, the brothers decided to kill the Rakshasa.
They cut off the arms of Viradha. The huge demon fell on the ground. The brothers wrestled and maimed the demon.
As he could not be killed by weapons they decided to strangle and bury him.
Viradha then realized that brothers before him where incarnation of Vishnu and Sesha. He then narrated his story. He was a Gandharva by the name of Tumburu and was cursed by Kubera to born as a Rakshasa. Kubera had told Tumburu that he will be relieved of his curse when Sri Ram will defeat him in a fight.
Viradha then gave the direction to Sri Ram to the ashram of Sage Sharabhanga and asked the Lord to bury him so that he will be relieved of the curse.
When he was buried, Viradha regained his true form of Gandharva.