1) Wikipedia (!)
Prabhasa, one of the eight 'Ashtavasus':
The legend behind Bhishma's birth is as follows — once the eight Vasus ("Ashtavasus") visited Vashishta's ashram accompanied by their wives. One of the wives took a fancy to Nandini, Vashishta's wish-bearing cow and asked her husband Prabhasa to steal it from Vashishta. Prabhasa then stole the cow with the help of the others who were all consequently cursed by Vashishta to be born in the world of men. Upon the Vasus appealing to Vashishta's mercy, the seven Vasus who had assisted in stealing Nandini had their curse mitigated such that they would be liberated from their human birth as soon as they were born; however, Prabhasa being protagonist of the theft, was cursed to endure a longer life on the earth. The curse, however is softened to the extent that he would be one of the most illustrious men of his time. It was this Prabhasa who took birth as Devavrata (Bhishma).
2) Vyasa Mahabharata:
'There was a king known by the name of Mahabhisha born in the race of Ikshvaku.
"One day the celestials had assembled together and were worshiping Lord Brahma. Many royal sages and king Mahabhisha also were present on the spot. And Ganga, the queen of rivers, also came there to pay her adoration to the Lord Brahma. And her garments white as the beams of the moon was displaced by the action of the wind.
And as her person became exposed, the celestials bent down their heads. But the royal sage Mahabhisha rudely stared at the queen of rivers. And Mahabhisha was for this cursed by Lord Brahma, who said, 'You hast forgotten yourself at the sight of Ganga, you will be re-born on earth.
"Vaisampayana continued, 'King Mahabhisha then recollecting all the monarchs and ascetics on earth, wished to be born as son to Pratipa of great prowess. And the queen of rivers, too, seeing king Mahabhisha lose his firmness, went away, thinking of him wishfully.
On her way, she saw those dwellers in heaven, the Vasus, also pursuing the same path. And the queen of rivers beholding them in the predicament, asked them, 'Why look ye so dejected? Ye dwellers in heaven, is everything right with you?'
Those celestials, the Vasus, answered her, saying, 'O queen of rivers, we have been cursed, for a venial fault, by the illustrious Vasishtha in anger. The foremost of excellent Rishis, Vasishtha, had been engaged in his twilight adoration and seated as he was, he could not be seen by us. We crossed him in ignorance. Therefore, in wrath he hath cursed us, saying, Be ye born among men!' It is beyond our power to frustrate what hath been said by that utterance of Brahma. Therefore, O river, thyself becoming a human female make us the Vasus, thy children. O amiable one, we are unwilling to enter the womb of any human female.' Thus addressed, the queen of rivers told them, 'Be it so and asked them, 'On earth, who is that foremost of men whom ye will make your father?'
"The Vasus replied, 'On earth, unto Pratipa shall be born a son, Santanu, who will be a king of world-wide fame.' Ganga then said, 'Ye celestials, that is exactly my wish which ye sinless ones have expressed. I shall, indeed, do good to that Santanu. That is also your desire as just expressed.' The Vasus then said, 'Please throw your children after birth, into the water, so that, we may be rescued soon without having to live on earth for any length of time.'
Ganga then answered, 'I shall do what ye desire. But in order that his union with me may not be entirely fruitless, provide ye that one son at least may live.'
The Vasus then replied, 'We shall each contribute an eighth part of our respective energies With the sum thereof, thou shall have one son according to thy and his wishes. But this son shall not beget any children on earth. Therefore, that son of thine endued with great energy, shall be childless.'