Shaantanu was the second son of King Prateep of Hastinaa Pur. His elder brother was Baahleek who retired to forest and his younger brother was Somdatt who was ruling some other region. Shaantanu was a very peaceful king. The legend has it that whoever touched him he also became peaceful and young that is why he was Shaantanu. In his previous birth he was a king named Mahaabhish.

Once Mahaabhish was sitting in Indra's court that Gangaa also came there along with her father Brahmaa Jee. By chance her garment was displaced by wind. All other celestials lowered their heads seeing this, but Mahaabhish kept looking at her. At this obscenity, Brahmaa Jee cursed both of them to be born on Prithvi until the King got angry with her. At the same time eight Vasu also had the Shaap of Vashishth to be born on Earth, so they requested Gangaa to be their mother with the condition that she will make them free from that human form as soon as they were born. Gangaa agreed with the condition that one of them will have to live on Prithvi. They agreed. Thus Mahaabhish decided to be the son of King Prateep to pass his time on Prithvi.

Since Baahleek went to forest, Shaantanu was crowned the King. Once Shaantanu had no rains in his kingdom. On asking the reason, Pandit told him that because his elder brother was still there, and he has not taken the kingdom lawfully from him that is why it is not raining. So he sent some ministers to Baahleek for his proper permission to rule the kingdom. Ministers changed his mind and Baahleek started abusing Ved. Since such a man could not be appointed as a king, Shaantanu became the official king and it rained.

Shaantanu and Gangaa
Once he went for hunting that he saw a beautiful young girl named Gangaa. He got attracted to her and asked her to marry him. She got ready to marry him but on two conditions, one that Shaantanu would never ask her what and why she was doing anything; and two, the day he would ask her this question, she would reply his question and leave him for good. Shaantanu agreed and they got married.

After some time Gangaa had a son. All got very happy to hear the news that now Hastinaapur had got its Crown Prince. Shaantanu had no limits of his happiness. But soon he plunged into the sea of grief when he saw that Gangaa was taking his son towards Gangaa River. He followed her for his curiosity sake, but he was stunned to see that she drowned him in the river. According to Gangaa's condition he could not ask her what and why she was doing that? She did the same with his next six sons. Everybody was talking about Raajaa that why Raajaa does not ask her about his sons.

Now she was to give birth his eighth child. This time also she gave birth to a son and as usual she took him to Gangaa River. But this time the King could not resist himself and asked her - "Gangaa, Why are you doing this to my sons? No mother does this to her children." Gangaa told him that she was living with him because of a curse, even he himself was living with her because of a curse. And as he had asked her this question, now she is free from this curse, so she has to go back. She told him that these children were also born to her because of Shaap. So she did not kill them rather she freed them from the Shaap. Since this child was still very young, she would take him with her, educate him and bring him back to the King when he would be worthy of becoming Crown Prince. She named him Devadatt, and she went away with the prince. Raajaa again plunged into the sea of grief, but could not do anything.

Some years passed, that one day King heard that a young boy has stopped Gangaa's water with his arrows. He got surprised to hear this and went to see who was such a skilled boy who had done this to Gangaa waters. When he came to the boy face to face, He saw Gangaa coming towards him. She introduced him to his father, handed him over to him and went away. Shaantanu got very happy to see his son, took him to the the city and declared him the Crown Prince.

Shaantanu and Satyavatee
Again some time passed and once again the similar situation arose before Shaantanu. Once Shaantanu went for hunting that he saw a beautiful girl who used to take passengers from this bank of Gangaa to the other bank of Gangaa. He fell in love with her. She was a fisherman's daughter. Her name was Satyavatee. Her father knew that she would live in palace and her children will rule th kingdom, but he did not know in whose palace. Shaantanu decided to propose for her, so he went to Daash Raaj (Satyavatee's father) and proposed for her. Satyavatee also loved him but she had no say of her own.

Daash Raaj said - "O King, I have no objection, but her birth chart says that she will rule the kingdom, and I don't see any chance of that, that is why I cannot marry her to you." Since Shaantanu had already declared Devadatt as the Crown Prince of Hastinaa Pur, he could not say anything and came back to his palace. But he had got lost in Satyavatee's thoughts. He did not come out of his room for several days. Devadatt noticed his condition and asked his ministers etc if anybody knew about its reason. All expressed their ignorance. Then he asked Raajaa's Saarathee (chariot driver) if he knew something about it. He told him the whole story.

Devadatt went to Daash Raaj and asked him why he could not marry his daughter to Shaantanu. Daashraaj told him also what he told Shaantanu also. He said - "I have no objection but Satyavatee's chart says that she would rule the kingdom, and Raajaa has already declared you as that Crown Prince, then her chart does not match with this condition, so I cannot marry her to him." Devadatt said - "I take vow that I leave my status of Crown Prince and I will never ask for kingdom. Only Satyavatee's son will be the King of this kingdom."

But Daash Raaj was still not satisfied with this vow, he said - "O Prince, It is all right that you have taken a vow, but what is the surety of your children? If they ask the kingdom after they grow up, then?" At this Devadatt took another vow - "I Devadatt take this vow that I will never marry my whole life, I will never have my own children, I will always live as a bachelor and I will die as a bachelor." and asked him - "Now O Daash Raaj, Is there any more problem in your mind?" There was no problem now so Devadatt took Satyavatee to Hastinaapur.

Because of taking this great vow, he was named as Bheeshm. Bheeshm means great and very difficult. Because he took such a great and very difficult vow, he was named as Bheeshm. He got so popular by this name that people forgot his original name. When Bheeshm brought Satyavatee home, Shaantanu got very angry at him on his this action, but got moved too seeing his respect and love towards himself. He bestowed him a boon of Ichchhaa Mrityu, that Death would not come to him itself until he wishes for it.

On one side Shaantanu was happy to get Satyavatee but on the other side he was dissatisfied thinking about injustice done to his eldest son Devadatt. He always thought about it. He had two sons from Satyavatee - Chitraangad and Vichitraveerya. After that he fell ill and  died soon leaving both sons young. Devadatt took care all of his affairs.