As the trio of brothers meditated dedicatedly, Indra, the king of Astral world got worried. Anyone who has higher capacity worried him as they could overthrow him and assume his position. So he created illusions in their mind that their entire kingdom was being burnt, family dragged and killed, civilians roasted.
Knowing these tricks of Indra, they did not give in and wake up from their continued meditation. They meditated one minded and finally Lord Brahma the creator of this particular world had to appear.
"Oh my stubborn children, I'm pleased with your efforts, what do you desire from me, I grant you each a boon." said father Brahma.
"I want to be immortal Sir." said Ravana.
"My dear Son, ask for 10000 years, or even million but not for immortality, you can ask who you don't want to die from. But not to die." said Brahma.
Ravana being the genius he is, thought for a while and then said "I want not to die from Devas (demigods), Yakshas, Gandharvas, Rakshasas and so on" They were the most powerful beings while humans were weak and which he neglected.
"TathAstu (So Be it)" said Lord Brahma.
"My Lord, there is pleasure in sleeping and eating, I want my joy in these two multiplied." asked KumbhaKarna. "TathAstu" said Lord Brahma.
"What do you want child? " he asked Vibhishana, the youngest of the three.
"I want to be righteous, bless me that I never give up Dharma even if I have to stand against my own people for the sake of it. Let me never be swayed away from Dharma" asked Vibhishana, like a true son of a sage, which he was.
"My Child you already have that, Now I'll Bless you with Immortality." said the Brahma and made him an immortal.
As destiny had it, Vibhishana later in life had to fight against his own brothers and kingdom for the sake of Dharma. He gave up kingdom, blood relatives for the sake of Dharma. Eventually he was crowned the king of Kingdom that his brother Ravana ruled.
