My question concerns vibhishana and sugreeva of ramayana. After Vibhishana was made the king of Lanka and Sugreeva returned to rishyamukh hill what happened to them after that and how did they die?
According to the Uttara Kand of Valmiki Ramayan, Vibhishana didn't die. He was commanded by Sri Rama to rule over Lanka. And Sugriva entered in Sarayu. It is described as:
Apprised of Rama's determination of going to heaven, Vanaras, born of the Devas, Rishis and Gandharvas taking Sugriva before them, came there to see Rama. And they all said "O Rama, O foremost of men, if thou dost repair to heaven forsaking us all, it will be hurling Yama s rod upon us." The highly powerful Sugriva too saluting Rama said: "O lord of men, having placed Angada on the throne I have come hither.
I have determined upon following thee, O King." Hearing the words of Vanaras, Rama said that their desire would be fulilled. He then said to Vibhishana the king of Rakshasas: As long as people shall breathe, O Vibhishana, O highly powerful lord of Rakshasas, thou shalt preserve thy body in Lanka.
As long as the moon, the sun and the earth shall exist and as long as my story shall remain current, thy kingdom shall be in existence. Thou didst, out of friendship, carry out my behests and perform my works. Do thou piously govern thy subjects.
I should not speak any thing else. What shall I say more unto thee, O highly powerful lord of Rakshasas. Worship Jagannatha (the lord of the world), the presiding deity of the Ikshwaku race.
As described in above paragraphs, Sugriva also want to take Jala Samadhi and Sri Rama has fulfilled his wish. And at last Sugriva soul has entered in the Sun.
The Vanaras and bears shall enter into their respective celestial forms. They shall enter into the Devas from whom they had respectively originated. Sugriva shall enter into the Sun s region.
In Mahabharata, Sabha Parva, it is described that Sahadeva has met Vibhishana while journeying for the purpose of Rajasuya Yagna.
And, O king of kings, that slayer of all foes, the virtuous and intelligent son of Madri having arrived at the sea-shore, then despatched with great assurance messengers unto the illustrious Vibhishana, the grandson of Pulastya. And the monarch willingly accepted the sway of the son of Pandu, for that intelligent and exalted king regarded it all as the act of Time. And he sent unto the son of Pandu diverse kinds of jewels and gems, and sandal and also wood, and many celestial ornaments, and much costly apparel, and many valuable pearls. And the intelligent Sahadeva, accepting them all, returned to his own kingdom.