Bhisma could not be killed unless he lowered his bow in battle. He would lower a bow only before a woman but no woman was allowed to enter the battlefield. Shikhandi provided the solution to this problem. He was born a woman, raised as a man, and later acquired a penis from a Yaksha. He was taken into battle as Arjuna’s body armor on Krishna’s chariot. Bhisma refused to fight him/her. Arjuna took advantage of this and shot a hundred arrows that pinned the great warlord to the ground. There was no way to kill the old man as he had the gift of choosing the moment of his death.
This article is also listed under Mahabharata