Bhishma's Terrible Vow
भीष्म प्रतिज्ञा
Quick Answer
Prince Devavrata took the fearsome vow of lifelong celibacy and renounced his claim to the throne of Hastinapura so that his father Shantanu could marry Satyavati. This supreme sacrifice earned him the name Bhishma and the boon of choosing his own time of death.