Bhishma
Janamejaya's ancestor Shantanu, the king of Hastinapura has a short-lived marriage with the goddess Ganga and has a heroic son, Devavrata (later to be called Bhishma). Many years later, when the king goes hunting, he spots Satyavati, the daughter of a fisherman, and wants to marry her.
But the fisherman refuses to consent to the marriage, saying that he is not assured of his daughter's happiness and status in the royal family. So he asks Shantanu to promise that her son (his grandson) should ascend the throne of Hastinapura. Shantanu promises that, but still the fisherman is not convinced.
He doubts Devavrata's renouncement of his right to the throne. So Devavrata takes an oath that he will never claim his right to the throne and will always support the one holding the position, no matter what.
The fisherman still doubts whether Devavrata's son would not claim his right to the throne.
Devavrata then vows to be a lifelong celibate, so that Satyavati's son can be the king.
Such a vow was unheard of amongst warrior dynasties, thus inspiring the name Bhishma —
'the person of the terrible oath'.
It was because of this oath that he later fights against the Pandavas, despite supporting them ideologically.