Story of Viradha is found in Ramayana when Sri Rama goes to Dandaka
 forest in his exile period.
Viradha was a huge demon. He had got a boon from God Brahma that no
 weapon would kill him. The boon made him arrogant and fearless. He roam
 around the world attacking all living beings. 
Viradha makes a sudden appearance before Sri Rama, Laxman and Sita.
 Before the brothers can understand what was happening, Viradha had
 snatched Sita.
Soon a fight started between Sri Rama, Laxman and Viradha. 
In a fit of rage, Viradha carried Sri Rama, Laxman and Sita, deep into the
 forest.
When they reached the dense forest, brothers decided to kill Viradha.
The brothers cut off the arms of Viradha. The demon finally fell on the
 ground.
As the demon could not be killed by weapons, the brothers decided to
 strangle and bury him. 
That time, he started to tell his story. 
Viradha was the gandharva by the name  Tumburu . 
He was cursed by Kubera to born as a demon. 
Kubera had said that Tumburu would be relieved of his curse when Sri
 Rama defeats him in a fight. 
When Viradha got buried, he regained his original form as the  gandharva.