Ravana expelled his younger brother Vibhisana from Lanka because he tried to discourage Ravana from fighting with Rama. Vibhisana decided to go Rama and help him.
Seeing Vibhisana, Sugreeva and the others grew suspicious, but Hanuman assured them of his honest nature. Like a loyal servant, Vibhisana told Rama about Ravana's strengths and weaknesses. He explained that Ravana had been granted the boon of immortality by Brahma. He gave Rama all the details of Ravana's weapons, soldiers, army, and methods of warfare. He also told them about Indrajit's ability to disappear while fighting the enemy.
All this valuable information helped Rama decide how to organise his army and fight Ravana. Finally, Vibhisana suggested that Rama should pray to Samudra to help him build the bridge to Lanka and cross the sea.
Rama warmly hugged Vibhisana and thanked him for all his help. Rama promised to crown Vibhisana the of Lanka after defeating Ravana and freeing Sita from his clutches.