Kumbhakarna undertook a penance to get the blessings of Lord Brahma. Brahma was pleased with Kumbhakarna and offered him a boon. But that time Saraswati tied his tongue firmly. So instead of asking for Indraasan (sit of Indra) Kumbhakarna uttered Nindraasan (bed for sleep). Brahma granted his request. Ravana realized that the boon was a curse in reality and so he asked Brahma to undo his boon. In another version of the story Kumbhakarna did another austerity to ask Brahma to take his boon back and make him alert always. Thus Brahma decided that Kumbhakarna would sleep for six months at a stretch and remain awake for another six consecutive months. However when Kumbhakarna woke up from six months sleep he felt very hungry and ate whatever he got in the vicinity including humans.
When Ravana was humiliated in the war with Rama he decided to seek his brother Kumbhakarna's help. When Kumbhakarna was informed he was preparing to go for sleep. In the other version of the story Kumbhakarna was made awakened from the sleep with a great difficulty. However Kumbhakarna had no mood to fight. He tried to pursue Ravana to stop the war as he thought Ravana was wrong and was inviting unnecessary trouble by his useless insistence to marry Sita. He also advised Ravana to release Sita and put an end to the conflict with Rama.
But Ravana insisted Kumbhakarna to join the war. Kumbhakarna was very loyal and patriotic. He was also boastful about glory of demon rule and prosperity of Lanka. Ravana knew this weakness of Kumbhakarna and hence he appealed Kumbhakarna in the name and honor of the Rakshasa clan and dynasty. Kumbhakarna was charged and agreed to terminate Rama forever. He was full of enthusiasm and energy and drank before leading the Rakshasa army in the battlefield.
A fierce fight started between the Rakshasa and Vanara army. Kumbhakarna killed and ate many monkeys. Even he injured Hanuman, knocked Sugreeva down to unconsciousness and imprisoned him. Therefore Rama took the charge of the battle by himself. Kumbhakarna had magical power and he was adept with many illusive forms to confuse the enemy. It was difficult to hit Kumbhakarna's true body as many Kumbhakarna appeared in front and all the missiles and arrow missed him.
Kumbhakarna engulfed a large portion of the monkey army and created 'Rain of fire'. This resulted in confusion and chaos amongst the monkey battalion. Rama in turn created 'rain of fire' to extinguish the fire. Kumbhakarna created 'rain of stones' and Rama countered the attack by creating an appropriate protective cover. The battle continued for quite a long time. Finally Rama attacked Kyumbhakarna with his most powerful arrow strengthened with deadly divine missile. When this missile hit Kumbhakarna he was heavily injured and his life came to an end.
On the deathbed Kumbhakarna uttered the name of Rama and thus he was liberated from the instincts and tendency of mortal world.
Kumbhakarna had two sons Kumbha and Nikumbha who also fought in the battle against Rama and were killed.