The eight vasus are the eight demi-gods Anala, Anila, Soma, Ahas, Dhara, Dhruva, Pratyoosha and Prabhasa. Together, they represent the very elements of the Mother Nature.
Once, the wife of Prabhasa persuaded her husband to get her a cow which actually belonged to Rishi Vashisht. Prabhasa was initially reluctant to ask for anything from a Brahmin but disarmed by the usual weakness of a man towards his wife, he agreed to steal the cow for her. With the help of his seven brothers, the eight vasus purloined away Vashisht’s cow for Prabhasa’s wife. Rishi Vashisht, through his ascetic powers, figured out that the stealth was an act of the vasus. He cursed the eight vasus to be born on earth as mortals and suffer the pain like all mortals do.
The eight vasus begged Rishi Vashisht for mercy, but as I mentioned here, a curse could not be taken back. It could, at best, be modified. Rishi Vashisht granted some relief to the seven vasus that their lifetime on earth would be short and after that they could ascend to paradise. But the eighth vasu, Prabhasa, who instigated the other seven to steal the cow out of his passion for his wife would suffer the pain of the account of all the seven vasus.
He who committed a crime out of desire for his wife will never know the pleasure of a woman. Even without a wife and sons, he will spend his entire life struggling to serve his household. And in the end he will die at the hands of a woman, for his desire for a woman made him turn vasus into criminals.
The eight vasus then went to the Mother of all beings, Ganga to help them keep their stay on earth short. Ganga, who too was cursed by Brahma to suffer as a mortal on earth promised them that she would give them birth through her own womb and will drown them in the waters of Ganga to set them free from the curse the moment they would be born. Ganga drowned the seven vasus immediately on their birth, but Rishi Vashisht’s curse came true when she was about to drown Prabhasa and she was stopped by Shantanu. It was Prabhasa who grew up as Devavrata, later to be known as Bheeshma for his terrible oath, the oath that will be the cause of all his suffering and also, in a way, the cause of the Kurukshetra war.