When his father could not marry a fisher-woman maiden named Satyavati, as her father wanted her sons to inherit the throne, Bhishma helped his father by renouncing his claim to the throne and taking a vow of celibacy. For this terrible oath, his father granted him the boon that death would not approach him till he himself desired it.
Because of his actions, Amba, the eldest princess of Kashi could not marry Salya, the man she chose. In desperation, she tried to persuade Bhishma to marry her, but her efforts were futile. At her behest, Parashurama tried to persuade his disciple by force of arms, but to no avail. Amba became his enemy.
Later, Amba obtained both a garland that would enable the wearer to kill Bhishma, as well as the boon that she would be the one who kills Bhishma in her next birth. In her next birth she was born as Shikandi, a daughter of Drupada, and was later turned into a man. (Read about these incidents here.)
When war was declared between his grand-nephews, the Pandavas and the Kauravas, Bhishma had to support the Kauravas despite his personal preference for the Pandavas, for his duty was to the throne, and to King Dhritharashtra, his nephew who ruled over Hastinapura.
By nearly unanimous consent, he was named the Generalissimo of the Kaurava forces. The lone dissenting voice was that of Karna who was suspicious Bhishma’s partiality for the Arjuna. After a heated exchange of words, Karna left the camp, and did not take part in the war till Bhishma’s death.
The battle ebbed and flowed as the days progressed. Bhishma wrought great havoc on the Pandava forces, with only Arjuna able to withstand his attack. On the tenth