Vasus are the eight attendant deities of Indra, the king of
Devas. There is a story in the Adi Parva of the Mahabharata in which the Vasus
are cursed by Vashisht Rishi (Sage Vasishta). The curse and story has the reason for the long life of Bhishma on earth.
One day the Vasus were roaming on the earth along with their
wives. They visited many places and finally arrived at the forest in which the
ashram of Vashisht Rishi was located.
One of the Vasus was Prabhasa. His wife happened to see sage
Vashishta's cow grazing. She was struck by the beauty of the cow and asked her
husband to get it.
Prabhasa was not ready to steal the cow of the sage but when
his wife kept pushing the matter, he relented. He along with his brothers stole
the cow.
On discovering the theft, the enraged sage cursed the eight
immortal Vasus to be born as mortals.
The eight Vasus were born as the sons of Ganga and King
Shantanu. Seven got relief from the curse when they were drowned by Ganga in
the river. But Prabhasa was not so lucky, for his sin he had to live a long
life on earth as Bhishma.