Ravana and his brothers

There was a Rakshasi called Kaikasi. She was the daughter of a great Rakshasa called Malyavan. At that time the devas were all powerful and the Rakshasas lived in hiding. One day Malyavan saw the wealth and strength of Kubera who was the son of a sage called Visravas. Coming back home he advised his daughter Kaikasi to approach Visravas and request him to marry her. Visravas refused but when she went on serving him and requested him, he agreed. Four children were born to them. They were Ravana, Kumbhakarna, Vibheeshana and a daughter called Surpanaka

As per the advice of Malyavan and Kaikasi Ravana, Kumbhakarna and Vibheeshana went to do Tapas (meditation and prayer) to Lord Brahma. After a long time Lord Brahma appeared before them and wanted to know what boon he can give them. Ravana requested that he should not be killed by anybody. Lord Brahma told him that, he could not give him that boon as it went against nature. Then Ravana asked him that he should not be killed by anybody except monkeys and man. Brahma gave him this boon. Kumbhakarna wanted a boon so that no one can defeat him (Nirdevathwam). But the devas made him ask for always sleeping (Nidrevathwam). Vibheeshana asked for Bhakthi to Lord Vishnu.