Who is Drona?

The poor Brahmin

Drona was a Brahmin, the son of the sage Bharadvaja, and a master of weapons. As a boy he studied alongside Prince Drupada, who promised to share his kingdom with him one day. Years later, poor and unable to give his son Ashwatthama even a cup of milk, Drona went to Drupada, now a king, for help. Drupada laughed: a king has no friends among beggars.

Drona never forgot it.

The teacher

Bhishma hired Drona to teach the princes of Hastinapura, and he became the most famous teacher in the land. His favourite was Arjuna, and he promised that no one would ever be a better archer. That promise is why he asked the forest boy Ekalavya for his right thumb.

When the princes finished their training, Drona asked for his fee: bring me King Drupada as a prisoner. Arjuna did. Drona took half of Drupada’s kingdom and let him go, and Drupada went home to pray for a son who would kill Drona. That prayer produced Dhrishtadyumna and Draupadi.

How Drona’s story ends (spoilers)

Drona fought for the Kauravas, and after Bhishma fell he commanded their army. No one could beat him. So the Pandavas spread word that Ashwatthama was dead. Bhima had killed an elephant of that name, and Yudhishthira, who never lied, confirmed it, adding "the elephant" under his breath. Drona put down his weapons in grief, and Dhrishtadyumna killed him.