Shikhandi (Sanskrit: शिखंडी, Śikhaṇḍī ; Indonesian, Srikandi) is a character in the Hindu epic, the Mahābhārata. He was originally born as a girl child named 'Shikhandini' to Drupada, the king of Panchala. Shikhandi fought in the Kurukshetra war on the side of the Pandavas, along with his father Drupada and brother Dhristadyumna.
He had been born in an earlier lifetime as a woman named Amba, who had won in swayamvara by Bhishma for his younger brother and later sent her back to her lover who insulted her and sent her back to Bhishma. Rejected by Bhishma for marriage, due to his oath of lifelong celibacy, and feeling deeply humiliated and wanting revenge, Amba carried out great prayers and penance with the desire to be the cause of Bhishma's death. Amba was then reborn as Shikhandini.
King Drupada knew who she was and she was sent on exile as king Drupada feared of fighting from Bhishma. She again did great prayers and gained manhood. In the battle of Kurukshetra, Bhishma recognised him as Amba reborn, and not wanting to fight 'a woman', lowered his weapons. Knowing that Bhishma would react thus to Shikhandi, Arjuna hid behind Shikhandi and attacked Bhishma with a devastating volley of arrows. Thus, only with Shikhandi's help could Arjuna deal a death blow to Bhishma, who had been virtually invincible until then.
Shikhandi was finally killed by Ashwatthama on the 18th day of battle

Source: http://en.wikipedia.org/wiki/Shikhandi