People of the epic · Mahabharata questions
Why did Bhishma take his vow?
So that his father could marry. Shantanu wanted Satyavati, whose father demanded that her children inherit the throne. Devavrata renounced the kingdom, and then, to remove any threat from his own sons, swore lifelong celibacy. For that terrible oath he was named Bhishma.
Devavrata was the son of Shantanu and the river goddess Ganga, raised by his mother, trained by Parashurama and Brihaspati, and installed as crown prince. Years later Shantanu fell in love with Satyavati, a fisherman daughter who ferried travellers across the Yamuna. Her father would not give her except on a condition: her sons, not Shantanu existing heir, must rule.
Shantanu would not ask it and went home silent and sick with wanting. Devavrata found out the reason, went to the fisherman himself, and gave up the throne. The fisherman objected that the prince own sons might one day reclaim it. So Devavrata took a second and far heavier vow: he would never marry and never father a child. The epic says the gods rained flowers and called him Bhishma, he of the terrible oath, and his father granted him the boon of choosing the hour of his own death.
Everything afterwards is the cost of that morning. Bhishma cannot inherit, so he installs and protects a sequence of weak kings. He carries off three princesses of Kashi for his half-brother and refuses to marry the eldest, Amba, when she is rejected by the man she loved; her rage takes her through a life of penance to rebirth as Shikhandi, the one by whose presence he will be killed. He is bound to the throne of Hastinapur rather than to justice, and so he sits silent while Draupadi is dragged into the hall, and fights for Duryodhana in a war he says openly is wrong.
The Mahabharata admires the vow and shows exactly what it costs. Bhishma dies on a bed of arrows telling Yudhishthira how a king should rule, having never ruled and having never once been able to act on his own judgement.