Ravana and Brothers Perform Austerities for Boons
Jealous of Kubera's prosperity, Ravana and his brothers undertake terrible austerities to win boons from Brahma. Ravana stands on one foot for a thousand years, then cuts off his own heads and offers them into the fire. Brahma grants him invincibility against all beings except men — and Ravana, in his arrogance, ignores the exception.
The sons of Vishrava saw Kubera seated with their father, surrounded by his great prosperity. Jealousy was born in them. They resolved to perform austerities — to win from Brahma what Kubera had won through his own devotion.
Dashagriva stood on one foot for a thousand years. He controlled himself and lived on the wind, amidst five fires — the sun above and four fires around him. Kumbhakarna lay down on the ground, controlled in his food and controlled in his vows. Vibhishana fasted and meditated, surviving on one dry leaf, performing severe austerities throughout the entire period. With delighted minds, Khara and Shurpanakha served them and protected them while they tormented themselves.
When one thousand years had passed, Dashanana cut off one of his heads and offered it into the fire. The lord of the universe was pleased at this. Brahma himself went there and asked them to refrain from austerities.
He tempted each of them separately with a boon. "Stop. I am pleased with you. Ask for boons, except for immortality, and I will give you whatever you ask for. All the heads that you have offered into the fire out of your great desire will be reunited with your body according to your wishes. There will be no disfigurement of your body and you will be able to assume any form that you desire. You will be able to vanquish your enemies in battle."
Ravana spoke first. "May I never suffer defeat at the hands of gandharvas, gods, asuras, yakshas, rakshasas, serpents, kinnaras, and demons."
Brahma said, "You will have no fear from all those you have mentioned, but for man. This is the way I have ordained it."
Dashagriva was satisfied. Because of his evil intelligence, the man-eating one ignored men.
Then the great grandfather addressed Kumbhakarna. But Kumbhakarna's reasoning was clouded by darkness. He asked for a long period of sleep. Brahma granted it.
Then he spoke to Vibhishana. "Ask for a boon. I am pleased with you."
Vibhishana replied, "May my mind not turn towards adharma, even in times of supreme difficulty. And may the brahmastra weapon — which cannot be instructed — become manifest in me."
Brahma said, "Though you have been born in a rakshasa womb, your intelligence does not turn towards adharma. I am granting you immortality."
Having obtained this boon, Dashagriva defeated the lord of riches in battle and won Lanka from him. Aranyaka Parva, Chapter 556