According to Wikipedia, Shantanu has two sons by Satyavati, Chitrāngada and Vichitravirya. Upon Shantanu’s death, Chitrangada becomes king. He lives a very short uneventful life and dies. Vichitravirya, the younger son, rules Hastinapura.
Meanwhile, the King of Kāśī arranges a swayamvara for his three daughters, neglecting to invite the royal family of Hastinapur. In order to arrange the marriage of young Vichitravirya, Bhishma attends the swayamvara of the three princesses Amba, Ambika and Ambalika, uninvited, and proceeds to abduct them. Ambika and Ambalika consent to be married to Vichitravirya.
The oldest princess Amba, however, informs Bhishma that she wishes to marry king of Shalva whom Bhishma defeated at their swayamvara. Bhishma lets her leave to marry king of Shalva, but Shalva refuses to marry her, still smarting at his humiliation at the hands of Bhishma. Amba then returns to marry Bhishma but he refuses due to his vow of celibacy. Amba becomes enraged and becomes Bhishma’s bitter enemy, holding him responsible for her plight.
Later she is reborn to King Drupada as Shikhandi (or Shikhandini) and causes Bhishma’s fall, with the help of Arjuna, in the battle of Kurukshetra.