Dronacharya, the royal preceptor of the Kuru dynasty, was killed by Dhristadyumna, the son of King Drupada and the brother of Draupadi, during the Kurukshetra war in the Mahabharata.
Dhristadyumna had been born with the sole purpose of killing Dronacharya, as King Drupada had sought revenge for his humiliation at the hands of Dronacharya. During the war, Dhristadyumna and his brother Satyajit attacked Dronacharya, and after a fierce battle, Dhristadyumna beheaded him.
The death of Dronacharya was a significant turning point in the Kurukshetra war, as it led to the demoralization of the Kaurava army, who considered Dronacharya to be one of their greatest warriors and strategists.