According to Mahabharata Serial shown on TV

Four years passed since Devavrat came into the life of Shaantanu. Once Shaantanu went for gaming spree. Shaantanu followed a deer and came to the banks of Gangaa River. He was smelling some sweet smell also. He went to trace its source. He went on and on and came to Yamunaa River. He found a beautiful young woman. That sweet smell was spreading from her body all around her. He went there and asked - "Who are you and who are you waiting for?" She said - "I am the daughter of Daash Raaj and my name is Satyavatee. I row the boat and I am waiting for the passengers who want to cross Yamunaa River." Shaantanu said - "I am Shaantanu, the king of Hastinaapur. Will you take me also across the river?" "Yes, why not" Shaantanu traveled in her boat for quite some time, then he could gather the courage to propose her for marriage. Satyavatee said - "This will be decided only by my father."

Shaantanu went to Daash Raaj's house. When Daash Raaj knew that Hastinaapur king Shaantanu had come to his house, he felt very fortunate and welcomed him heartily. On asking as why did he come there, Raajaa proposed for marriage. Daash Raaj said - "I knew that you would come." "How" "Because Satyavatee's birth chart says that she will rule the kingdom. And nowadays who else is greater Raajaa than you." Raajaa said - "It means that I should take it as "Yes"." Daash Raaj said - "But Mahaaraaj, Satyvatee's fate line says this also that her children will rule the kingdom."

Shaantanu said - "But you know that I have already declared Devavrat the Crown Prince of Hastinaapur." "That is why I am putting this restriction." Shaantanu said - "But this is not a restriction like restriction. This is not a true restriction. I cannot do injustice with my eldest son. If you say then I can do anything else, but this injustice? No, no Daash Raaj, it cannot happen." Daash Raaj said - "You are ready to do all that which I do not want you to do, and you are not ready to do that one thing which I want you to do. You want to change the fate line of Satyavatee for which I am not ready. If the children of Satyavatee will have to rule, then her fate line itself will bring such a Raajaa here.

Shaantanu said - "Then you wait for such a candidate, because Shaantanu of Bharat Vansh cannot do injustice with one person by snatching his rights and giving them to another person." And Shaantanu came back to his palace. He was very sad and worried. After that Raajaa used to go somewhere for the whole day and come back in the evening tired and worried.

As Devavrat saw him worried and lost he asked him about the reason of his worry, but Raajaa didn't come out. Then Devavrat asked his Saarathee. After requesting a lot he told him that Raajaa used to watch Daash Raaj's daughter from behind trees, on the banks of Yamunaa the whole day. Devavrat asked him to get  his chariot ready. He went to Daash Raaj's house. Daash Raaj told him that the king wanted to marry his daughter. "Then what was the problem in it? Is he not worthy for your daughter?" "What are you talking about? In fact Satyavatee's fate line says that her son will rule a kingdom and Mahaaraaj didn't promise for it."

Devavrat said - "He could not have promised for that, because he had no right to promise for that. He has given this right to me, because he has already given me that royal throne. If I want I can give that throne to anybody, therefore I promise you that my royal throne will go to the eldest son of Satyavatee. So if you permit, can I take her now?" Daash Raaj said - "Not yet." "Now what?" Daash Raaj said - "As Raajaa didn't have the right to promise me on behalf of you, in the same way you also have no right to promise me on behalf of your descendents."

Devavrat said - "If I have no right to promise you on behalf of my descendents, then I finish all this conflict in one moment. At least I have this right that I can live childless. I take a vow with the witness of all four directions and Prithvi that I will never marry, remain Brahmchaaree for the whole life, live without my family line (children) and die without having any child."

Everybody including Devtaa got stunned hearing this vow of Devavrat. Devtaa declared - "You are great, only you could take such a vow. Because of taking such a Bheeshm (difficult) vow, you will be known as Bheeshm in Tri-Lok from today." They showered flowers on him and went to their Lok. Now Daash Raaj had no alternative except to send Satyavatee along with Devavrat.

Because of this vow Devavarat became Bheeshm, and now nobody knows his real name. Bheeshm came back to his camp with Satyavatee and told Shaantanu that he had brought her as his mother. Then he said - "Father, You had all the rights to promise Daash Raaj that only Satyavatee's eldest son will be the king of Hastinaapur, why didn't you do so? And for the same reason now I have vowed to remain Brahmchaaree lifelong."

Shaantanu got extremely sad hearing this and said - "Devavrat, We have got this tradition from our ancestor Bharat that the king is appointed from his Karm (actions) not by birth. And whoever is not even born yet, what is his Karm?" Bheeshm said - "But one duty of a Raajaa is this also that he should protect his public's rights." Now Raajaa got shocked and asked - "Putra, Whose rights I have not protected?" "Mine" "How?" Bheeshm said - "You have not protected my one right, that is "to make promise"."

Shaantanu said - "Putra, You have done that which will be remembered by Tri-Lok. For this, your father gives you this blessing and this Vardaan that until you wish yourself, you will not leave this Mrityu Lok, your name will be immortal, you will live until you will want to live, and Mrityu (death) will not come to you until you wish for it."

Shaantanu again said - "But I have a worry, if Satyavatee's son would not have been a worthy king, then what will happen to this kingdom?" Bheeshm said - "You need not to worry about it. For it I also take vow that whoever will sit on the throne, I will see the image of my father in him and serve him in the same way as I would have served my father. And I will not leave this Lok until Hastinaapur is secured from all sides." Shaantanu got very pleased hearing this vow.