Seeing Shantanu in a distressed condition, Devavrata was heartbroken and wanted to help his father and fulfil his wish so decided to take matter in his own hands.
Devavrata headed to Dasharaja’s hut and requested him to give his consent for the marriage, but Dasharaja put forth the same condition. Upon hearing the condition, Devavrata made a vow, that he would never become the king of Hastinapur and would be as royal guard to the sons of Satyavati who would succeed the throne after his father. He also vowed to never marry and never have kids, to prevent any disputes regarding the rights to the throne. Because of his terrible vow, he was given the name Bhisma (one who has taken a terrible vow), and when Shantanu heard the vow Bhisma made, he gave Bhisma a boon, that only when he wishes to die, he would die in the way he wanted.
After the vow was made, Dasharaja was pleased and agreed to the marriage of the couple, Satyavati became the Queen of Hastinapur and they were blessed with two sons, Chitrangada and Vichitravirya.