Originally known as Devavrata, which was the name given to him at birth, and Gangaputra, Bhishma is said to have taken his oath in order to allow his father, Shantanu, to marry a fisherwoman named Satyavati. The vow of celibacy would ensure that Satyavati’s children would be rulers, rather than Bhishma’s children. Bhishma also promised never to stake a claim to the throne himself. He was immediately recognized by the gods for this act and his own father granted him the power to choose the time of his own death. This was considered to be a greater gift than immortality, which would ultimately be a curse and lead to more suffering.
In one tale from of the Mahabharata, Bhishma is forced to battle with his guru, Parasurama, in order to honor his vow. He eventually does with Parasurama’s blessing. The pair battle for 23 days, each too powerful to defeat the other, until the gods decided it a draw.
Bhishma was such a powerful warrior that he could not be defeated until he delibertaely gave Arjuna a clue which would lead to his own death. With this clue, Arjuna was able to shoot arrows at Bhishma, who had laid down his own weapons, until they pierced his whole body. When he fell, Bhishma’s body was held off the ground by the bed of arrows protruding from his back. Even the gods were humbled by this sight and they blessed him.