Vibhishan who helped Lord Rama in the battle against Demon Ravana. Ravana abducted Lord Rama’s wife Mata Sita and Vibhishan was against this act. Thus Vibhishan helped Lord Rama to end Ravana by revealing the secret to killing Ravana. Because of these good deeds, Vibhishan was awarded immortality to guide people on the right path all over the world.
The hidden reason behind Vibhishana being immortal is that he knows the right way of how to govern the people.
Secret scroll of Rajaneethi: This book deals with techniques of propaganda and psychological warfare. “The most dangerous of all sciences is that of moulding mass opinion because it would enable anyone to govern the whole world,” - Vibhishana