Abhimanyu was the son of Arjuna and Subhadra (Krishna's sister), making him both Arjuna's son and Krishna's nephew. He learned the art of entering the Chakravyuha (war formation) while in his mother's womb, but not how to exit it. This knowledge gap led to his tragic death when he was trapped alone in the formation on the 13th day of the Kurukshetra war.