Kumbhkarn was the younger brother of Raavan and the elder brother of Vibheeshan. (Read about his birth in Raavan - Raavan (1) and Raavan (2).

Once all three brothers went to do Tap, he also did Tap for 10,000 years by burning himself in Panchaagni in summer, standing in torrential rains in rainy season and sitting in cold chilly water in winter season. But when the time came to get Var from Brahmaa Jee, Devtaa requested Him to give him sleep because he was troubling Devtaa and Rishi Muni very much. So Brahmaa Jee looked at Saraswatee Jee and Saraswatee Jee immediately sat on his tongue and she spoke - "Let me sleep for years together." Brahmaa Jee immediately said - "So be it". And after giving this Var Brahmaa Jee left immediately. When Kumbhkarn woke up from his trance he repented what he had asked for Var? Then Raavan requested Brahmaa Jee to change it to 6 months sleep and one day to eat, which He did. Raavan married Kumbhkarn to Virochan's granddaughter, Bali's daughter  Vajrajwaalaa.

One day Roopvatee, sent by Brahmaa as his sleep, came to him and he started feeling sleepy. So he said to his brother - "Brother, Get me a beautiful and comfortable house, I am feeling sleepy. So Raavan ordered his architects to build a palace for him. They built a beautiful and most comfortable palace for Kumbhkarn - one Yojan wide and two Yojan long. And Kumbhkarn went to sleep in that palace.

When Raavan had war with Raam, he woke him up with a great difficulty. After he woke up, he also said to Raavan - "Why did you think of fighting with Raam who is the Swaamee of Trilok? Why do you want to destroy the Raakshas community? Even now if you return His wife He will pardon you." Raavan scolded him - "I have not awaken you to hear all this sermon, I have awaken you to fight for me." Kumbhkarn understood that there was no use to say anything to him. So he got ready for war and fought very bravely.

As he arrived in battlefield, Vibheeshan greeted him and asked his forgiveness for leaving Raavan. Kumbhkarn blessed him that it was his good luck that he was in the service of Raam. He was killed by Raavan and got Mukti (salvation) from his second life on this Earth. Read about this incident here in "Raam"