King Shantanu saw a beautiful woman on the banks of the river Ganges (Ganga) and asked her to marry him.
She agreed but with one condition: that Shantanu would not ask any questions about her actions.
Shantanu accepted her condition and made a vow never to ask her the reason for her actions.
They married and she later gave birth to a son but drowned the child very soon after his birth.
Shantanu could not ask her the reason, because of his promise, she would leave him.
One by one, seven sons were born to them and were all drowned by Ganga.

When Ganga was about to drown the eighth son, Shantanu, devastated, could not restrain himself and confronted her.
Finally, Ganga explained to King Shantanu about Brahma's curse given to Mahabhisha and her.
Then, she told him that their eight children were Eight Vasus who were cursed by the renowned sage Vasishtha to be born on earth as mortal humans however when they pacified him, he limited his curse and told them that they would be freed from this curse within a year of their birth as humans.
So, she released seven of them from this life by drowning them all.
However the eighth child Bhishma, the amsha of Prabhasa was destined to live a long life and to never have a wife or have children.
However, the sage also gave a boon to him that he would be virtuous, conversant with all the holy scriptures and will be an obedient son to his father.

Ganga told Shantanu that she will take him to the heavens to train him properly for the King's throne and status following his birth.
With these words she disappeared along with the child while Shantanu was struck with grief thinking about spending the rest of his life without her.

The history 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 Kamadhenu, 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 incarnate in the world of humans.

Upon the Vasus appealing to Vashishta's mercy, the seven Vasus who had assisted in stealing Kamadhenu had their curse mitigated such that their avatars 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 his avatar would be one of the most illustrious men of his time.
It was this Prabhasa also called Vasu Dyaus whose ansha took birth as Bhishma.
Vishnu was in Vaikuntha when krishna was born.
It was a part of him that took birth as Krishna.
Similarly, part of Prabhasa took birth as Bhishma and his 7 brothers were part of the 7 vasus.

After Devavrata was born, his mother Ganga took him to different realms, where he was brought up and trained by many eminent sages (Mahabharata Shanti Parva, section 38).

Brihaspati: The son of Angiras and the preceptor of the Devas taught Devavrata the duties of kings (Dandaneeti), or political science and other Shastras.

Shukracharya: The son of Bhrigu and the preceptor of the Asuras also taught Devavrata in political science and other branches of knowledge.

Vashishtha, the Brahmarshi and Chyavana, the son of Bhrigu taught the Vedas and Vedangas to Devavrata.

Sanatkumara: The eldest son of Lord Brahma taught Devavrata the mental and spiritual sciences.

Markandeya: The immortal son of Mrikandu of Bhrigu's race who acquired everlasting youth from Lord Shiva taught Devavrata in the duties of the Yatis.

Parashurama: The son of Jamadagni of Bhrigu's race and the pupil of Shiva.
Parashurama trained Bhishma in warfare.

Indra: The king of the Devas.
He bestowed celestial weapons on Bhishma.
King Shantanu saw a beautiful woman on the banks of the river Ganges (Ganga) and asked her to marry him. She agreed but with one condition: that Shantanu would not ask any questions about her actions. Shantanu accepted her condition and made a vow never to ask her the reason for her actions. They married and she later gave birth to a son but drowned the child very soon after his birth. Shantanu could not ask her the reason, because of his promise, she would leave him. One by one, seven sons were born to them and were all drowned by Ganga. 
When Ganga was about to drown the eighth son, Shantanu, devastated, could not restrain himself and confronted her. Finally, Ganga explained to King Shantanu about Brahma's curse given to Mahabhisha and her. Then, she told him that their eight children were Eight Vasus who were cursed by the renowned sage Vasishtha to be born on earth as mortal humans however when they pacified him, he limited his curse and told them that they would be freed from this curse within a year of their birth as humans. So, she released seven of them from this life by drowning them all. However the eighth child Bhishma, the amsha of Prabhasa was destined to live a long life and to never have a wife or have children. However, the sage also gave a boon to him that he would be virtuous, conversant with all the holy scriptures and will be an obedient son to his father. 
Ganga told Shantanu that she will take him to the heavens to train him properly for the King's throne and status following his birth. With these words she disappeared along with the child while Shantanu was struck with grief thinking about spending the rest of his life without her.
The history 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 Kamadhenu, 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 incarnate in the world of humans.
Upon the Vasus appealing to Vashishta's mercy, the seven Vasus who had assisted in stealing Kamadhenu had their curse mitigated such that their avatars 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 his avatar would be one of the most illustrious men of his time. It was this Prabhasa also called Vasu Dyaus whose ansha took birth as Bhishma. Vishnu was in Vaikuntha when krishna was born. It was a part of him that took birth as Krishna. Similarly, part of Prabhasa took birth as Bhishma and his 7 brothers were part of the 7 vasus.
After Devavrata was born, his mother Ganga took him to different realms, where he was brought up and trained by many eminent sages (Mahabharata Shanti Parva, section 38).
Brihaspati: The son of Angiras and the preceptor of the Devas taught Devavrata the duties of kings (Dandaneeti), or political science and other Shastras.
Shukracharya: The son of Bhrigu and the preceptor of the Asuras also taught Devavrata in political science and other branches of knowledge.
Vashishtha, the Brahmarshi and Chyavana, the son of Bhrigu taught the Vedas and Vedangas to Devavrata.
Sanatkumara: The eldest son of Lord Brahma taught Devavrata the mental and spiritual sciences.
Markandeya: The immortal son of Mrikandu of Bhrigu's race who acquired everlasting youth from Lord Shiva taught Devavrata in the duties of the Yatis.
Parashurama: The son of Jamadagni of Bhrigu's race and the pupil of Shiva. Parashurama trained Bhishma in warfare.
Indra: The king of the Devas. He bestowed celestial weapons on Bhishma.
Bhishma