Who is Shikhandi?

Amba

The story begins a generation earlier. Bhishma carried off three princesses of Kashi to marry his young half-brother. The eldest, Amba, was already promised to King Shalva, and asked to go to him. Bhishma let her go, but Shalva refused her: she had been taken by another man. Bhishma, bound by his vow, could not marry her either.

With nowhere left to turn, Amba blamed Bhishma for ruining her life. She swore she would be the cause of his death, and gave up her life in fire to be born again for that purpose.

Born again

She was reborn as the daughter of King Drupada of Panchala, named Shikhandini, and raised as a son. Later, with the help of a nature spirit who exchanged forms with her, she became a man: Shikhandi.

Bhishma knew who Shikhandi had been. He had sworn never to fight anyone who had been born a woman.

How Shikhandi’s story ends (spoilers)

On the tenth day of the war, Shikhandi rode in front of Arjuna’s chariot. Bhishma lowered his bow, and Arjuna’s arrows brought the grandsire down. Amba’s vow was fulfilled. Shikhandi survived the battle, only to be killed in Ashwatthama’s night attack on the Pandava camp.