In the epic Mahabharata, Shantanu was a Kuru king of Hastinapura. He was a descendant of the Bharata race, of the lunar dynasty and great-grandfather of the Pandavas and Kauravas. He was the youngest son of King Pratipa of Hastinapura and had been born in the latter's old age. The eldest son Devapi had leprosy and gave up his inheritance to become a hermit. The middle son Bahlika(or Vahlika) abandoned his paternal kingdom and started living with his maternal uncle in Balkh and inherited the kingdom from him. Shantanu became the king of Hastinapura by default.
He is best known for being the father of Bhishma, one of the mightiest warriors of all time.
Brahma's Curse & Birth Of Shantanu
In his previous birth, Shantanu was a powerful king of the Ikshvaku dynasty named Mahabhisha. Mahabhisha possessed many virtuous qualities, and after performing a thousand Ashwamedha Yagnas (Horse Sacrifices) and a hundred Rajasuya Yagnas (to qualify as emperor), he had attained heaven after his death. Once he got an opportunity to visit the court of Brahma where all the Devas and Ganga were also present. While the celestials were worshipping Brahma, a wind blew and displaced Ganga's clothes revealing her body. Everybody present there bashfully bent their heads except Mahabhisha who kept on gazing at her body. Ganga was flattered by this and she also desired Mahabhisha (who was a partial incarnation of her husband, Varuna). Upon seeing this act, Brahma lost his temper and cursed him and Ganga to be born as mortals and that Ganga will cause much emotional pain to him. He also said that he will be only freed from this curse when he becomes angry upon Ganga's deeds. Mahabhisha then requested Brahma to be born as the son of Kuru king Pratipa and his wish was granted by Brahma. The eight Vasus who had been cursed by the sage Vashistha to born as mortals on earth as a punishment for stealing his divine cow approached Ganga. They requested to be born as her and Shantanu's sons and also requested her to kill them. Ganga agreed upon the condition that at least one child might live. They accepted her condition and told her that they will all contribute an eighth part of their energies to that child and he will be remain childless for his whole life. The Kuru king Pratipa was once meditating. At that time Ganga took the form of a beautiful woman approached the king and sat on his right thigh. When he asked her what did she want, Ganga requested him to become her husband. Pratipa however refused since he had taken a vow not to lust for anybody and that she had sat on his right thigh and according to traditions a man's right thigh was for his daughter or daughter-in-law while the left thigh was for his wife. He then proposed her for marrying his son to which she agreed. She also said that his son should never question her acts. A child was born to Pratipa and his wife in their old age. He was named Shantanu because when he was born his father had controlled his passions by ascetic penances. When he grew up into youth, his father told him that if he meets a celestial damsel in secret who asks to marry him, he should agree and he should never question her acts. Pratipa then installed Shantanu as king of Hastinapura and retired into the woods to perform penances. Bahlika who was elder than Shantanu also gave permission to him for becoming the king of Hastinapura.
Shantanu & Ganga
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. They married and she later gave birth to a son. But she drowned the child. Shantanu could not ask her the reason, because of his promise, lest she would leave him. One by one, seven sons were born to them and were 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 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 the seven of them from this life by drowning them all. However the Vasu Dyaus was cursed to live a long life and to never have a wife or have children. But 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. that she will take him to the heavens to train him properly for the King's throne and status. 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.
Shantanu Reunites With His Son
One day while hunting a deer along the banks of Ganga, Shantanu saw that the river had become shallow. While searching for the cause of this phenomena, he came across a handsome young boy who had checked the river's flow with his celestial weapon. The young boy was his son however he didn't recognize this because of only having seen him for a few moments after his birth. The boy recognized that he was his father however didn't reveal it to him instead disappearing in his very sight using his powers of illusion. Shantanu upon seeing this wondered whether the boy was actually his son and called upon Ganga to show the boy to him. Ganga thus appeared. She then revealed to him that the boy was actually his son Devavrata and that he was taught the knowledge of the holy scriptures by the sage Vasishtha and the art of warfare by Parshurama. After revealing the truth about Devavrata she told Shantanu to take him back with to Hastinapura. Upon reaching the capital Shantanu crowned Devavrata as the heir-apparent to the throne.
Shantanu & Satyavati
Four years later, Shantanu while travelling near the banks of Yamuna smelled a sweet scent coming from an unknown direction. While searching for the cause of the scent, he came across Satyavati from whom the scent was coming. Satyavati was an adopted daughter of the chief of the fishermen of her village. Upon seeing her, Shantanu fell in love with her and desired to her. Upon asking for his consent, her father agreed to the marriage on condition that Satyavati's son would inherit the throne of Hastinapura.
King Shantanu was unable to give his word on accession as his eldest son Devavrata was the heir to the throne. However, Devavrata came to know about this and for the sake of his father, gave his word to the chief that he would renounce all his claims to the throne, in favour of Satyavati's children. To reassure the skeptical chief, further he also vowed lifelong celibacy to ensure that future generations borne of Satyavati would also not be challenged by his offsprings. Upon hearing this vow he immediately agreed to the marriage of Satyavati and Shantanu. Devavrata was named as Bhishma(one who has taken a terrible vow) by the celestials because of the terrible oath he took. Upon returning with Satyavati to Hastinapura he told about his vow to his father. Upon hearing about this Shantanu became highly impressed and gave him a boon that he will only die if he chooses so. Shantanu and Satyavati went on to have two sons, Chitrāngada and Vichitravirya. After Shantanu's death, Chitrangada became the king of Hastinapura.