Droṇa (Sanskrit: द्रोण, Droṇa) was royal preceptor to the Kauravas and Pandavas; an avatar of Brihaspati. He was the son of rishi Bharadwaja and a descendant of sage Angirasa. He was a master of advanced military arts, including the divine weapons or Astras. Since Droṇa was not born from a womb, but from a vessel made of leaf, he was named 'Droṇa' which means 'vessel made of leaf'.

Once Rishi Bharadwaj went with his companions to the Ganga River to perform his ablutions. There he was beheld by the beauty of a beautiful Apsara called Ghritachi who had come there to bathe. The sage was overcome by desire, which caused him to produce semen involuntarily out of the visual excitation. Bharadwaja rishi captured this semen in a vessel called a Droṇa, and Droṇācharya himself sprang from the semen thus preserved. Droṇācharya spent his youth in poverty, but studied Dharma and military arts such as archery, in which he gained expertise, together with the then prince of Panchala, Drupada in the gurukul of Rishi Bharadwaja. Drupada and Droṇācharya became close friends.

Droṇācharya married Kripi, the sister of Kripa, who was the royal teacher of the princes of Hastinapura. Like Drona himself, Kripi and her brother had not been gestated in a womb, but outside the human body. Kripi and Droṇa had a son, Ashwatthama; Droṇa did penance so that his son would be as valiant as Shiva.

Drona approached Parasurama when he learnt the sage was giving away his possessions to brahmins. However, by the time he was approached by Drona, Parasurama only had his weapons left to give away. He offered to give Droṇa the weapons as well as the knowledge of how to use them. This is how Droṇa obtained the greatest weapons in his possession, and also his title of 'ācārya'.

For the sake of his wife and son, Droṇa desired freedom from poverty. Remembering a childhood promise given by Drupada, he decided to approach him to ask for help. However, King Drupada refused to even acknowledge their friendship, saying friendship was possible only between persons of equal stature in life. As a child, he said, it was possible for him to be friends with Droṇa, because at that time they were equals. But now Drupada had become a king, while Droṇācharya remained a luckless indigent. However, he said he would satisfy Droṇācharya if he asked for alms befitting a Brahmin, rather than claiming his right as a friend. Droṇa went away silently, but in his heart he vowed revenge.

Drona decides to continue Parashurama's legacy by starting his own school. He uproots his family and begins wandering Northern India. While at Hastinapur, he comes across the Kuru princes at play, and is able to use his abilities to help the princes solve some of their problems. Amazed, the princes go to their patriarch Bhishma with news of this magician. Bhishma instantly realized that this was Drona, and asked him to become the Guru of the Kuru princes, training them in advanced military arts. Drona's school soon accepted all students of Hastinapur and its allies. Many princes came to study under him.

Of all the Kaurava and Pandava brothers training under Drona, Arjuna emerged as the most dedicated, hard-working and most naturally talented of them all, exceeding even Drona's own son Ashwatthama. Arjuna assiduously served his teacher, who was greatly impressed by his devoted pupil. Arjuna surpassed Drona's expectations in numerous challenges. As a reward, Drona gave Arjuna mantras to invoke the super-powerful divine weapon of Brahma known as Brahmāstra, but told Arjuna not to use this invincible weapon against any ordinary warrior.

When Arjuna, inspired by his brother Bhima's nocturnal eating, mastered archery in absolute darkness, Drona was moved. Drona was greatly impressed by Arjuna's concentration, determination, and drive, and promised him that he would become the greatest archer on earth. Drona gave Arjuna special knowledge of the divine Astras. Drona was partial especially to Arjuna and Ashwatthama. Drona dearly loved his son Ashwatthama and as a guru, he loved Arjuna more than anyone.

A strong criticism of Dronacharya is due to his pervert behavior towards Ekalavya and his strong bias in favor of Arjuna. Ekalavya was the son of a Nishadha chief, who came to Dronacharya for instruction. Dronacharya refused to train him along with the Kshatriya princes because Ekalavya was not a Kshatriya prince. Ekalavya began study and practice by himself, having fashioned a clay image of Dronacharya. Solely by his determination, Ekalavya became a warrior of exceptional prowess. One day, Kuru princes' dog's barking disturbed a focused Ekalavya. Without looking, Ekalavya fired arrows that sealed up the dog's mouth without spilling any blood of dog or causing any injury to the dog. The Kuru princes saw this dog running back to them, and wondered who could have done such a feat. They saw Ekalavya, who announced himself as a pupil of Drona. Every person saw his skill in shutting the dog's mouth. But Drona had given promise that Arjun will be the greatest archer ever in the world, so he demanded Ekalavya cut his right hand thumb as fee of being teacher.

Drona held the invincible sword of Lord Brahma. Bhishma once told the story of this sword to Pandava prince Nakula. This sword was the primordial weapon created by the gods for the destruction of evil. The name of the sword was Asi, the personification and the primary energy behind all the weapons ever created. As per Bhishma, the constellation under which the sword was born is called Krittika, Agni is its deity, Rohini is its Gotra, Rudra is its high preceptor and whoever holds this weapon obtains victory for sure.

Dronacharya had been the preceptor of most kings involved in the Kurukshetra War, on both sides. Dronacharya strongly condemned Duryodhana exiling the Pandavas, as well as the Kauravas' general abuse towards the Pandavas. But being a servant of Hastinapura, Dronacharya was duty-bound to fight for the Kauravas, and thus against his favorite Pandavas. After the fall of Bhishma on the tenth day, he became the Chief Commander of the Kuru Army. Duryodhana manages to convince Drona to try and end the war by capturing Yudhishthira. Though he killed hundreds and thousands of Pandava soldiers, Drona failed to capture Yudhishthira on days eleven and twelve of the war, as Arjuna was always there to repel his advances.

On the 13th day of battle, Dronacharya formed the Chakravyuha strategy to capture Yudhishtira, knowing that only Arjuna and Krishna would know how to penetrate it. The Trigartas were distracting Arjuna and Krishna into another part of the battlefield, allowing the main Kuru army to surge through the Pandava ranks. Unknown to many, Arjuna's young son Abhimanyu had the knowledge to penetrate the formation but did not know the way out. At the request of Yudhishthira, Abhimanyu agreed to lead the way for the Pandava army and was able to penetrate the formation. However, he was trapped when Jayadratha, the king of Sindhu, held the Pandava warriors who were following him, at bay. Abhimanyu did not know how to get out of the Chakravyuha, but embarked upon an all-out attack on the Kuru army, killing tens of thousands of warriors single-handedly. Drona is impressed with Abhimanyu and praises him endlessly, earning the ire of Duryodhana. With his army facing decimation, and spurred on by Duryodhana's criticisms, Drona asked the Kaurava maharathis to simultaneously attack Abhimanyu, to strike down his horses and his charioteer and to disable his chariot from different angles. Left without support, Abhimanyu began fighting from the ground. Exhausted after his long, prodigious feats, Abhimanyu was eventually killed. After that, several who fought against Abhimanyu were criticized for their murder, such as Bhurishrava, Drona and Karna.

The devious murder of his son enraged Arjuna, who swore to kill Jayadratha the next day or immolate himself. Drona constructed 3 combined vyuhas to protect Jayadratha, first was the Shakata vyuha then was Padma Vyuha and last was the Srigantaka vyuha and at its rear was Jayadratha and stood at the head of the box formation or shakata vyuha. In the early part of the day, Arjuna and he duel, and Arjuna is unable to bypass his preceptor. With Krishna's prodding, Arjuna circumvents Drona. When Duryodhana rages at Drona, Drona replies and that he intends to capture Yudhishthira while Arjuna is away and would only hasten their victory. In a notable battle, Drona attempts to capture Yudhishthira but is stopped by Dhristadyumna. Drona severely wounds his friend's son, disarming him and forcing him to retreat. When he attempts to chase after Dhristadyumna, he is checked by Satyaki, who insults his teacher's teacher and issues a challenge. Their combat is described as fierce and despite being able to hold off Drona for several hours, Satyaki eventually tires and has to be rescued by the Upapandavas. Later in the day, Yudhishthira sends Satyaki to aid Arjuna. When Satyaki comes upon Drona, he circumvents him, saying he must follow in his teacher's footsteps. When Yudhishthira later sends Bhima, Drona recounts what happened with Arjuna and Satyaki, and hence makes sure he does not allow Bhima also to circumvent him. Angrily rebuking him, Bhima shatters Drona's chariot with his mace. Drona takes up another chariot, only for Bhima to smash that one as well. In total, Bhima smashes eight of Drona's chariots and is able to bypass his guru.

On the 14th night (as the Kauravas were not ready to stop) of the Mahabharata war, Drona is instigated by Duryodhana's remarks of being a traitor as he was not able to protect Jayadratha. Sensing his end is near, he used the Brahmastra against the common Pandava soldiers. At that moment, all the Sapta Ṛishis appeared on the sky and requested Drona to retract this ultimate weapon used on ordinary soldiers. Dronacharya obeyed, retracting the weapon. The Rishis continue and berate Drona for violating the rules of war, criticizing him for using divine weapons so indiscriminately. Drona reiterates that he is sworn to do all he can to protect Hastinapur, and that, moreover, he wants to do so for all that Dhritarashtra has given him. On that day, Drona kills many Pandava soldiers, including Virata in arrow-play and Drupada in a sword fight. Lamenting the deterioration of their friendship, Drona pays his respect to Drupada's corpse.

Knowing it would be impossible to defeat an armed Drona, Krishna suggested the Pandavas a plan to disarm their teacher. His idea was that Bhima first kill an elephant named Ashwatthama, and then claim to Dronacharya that he has killed Dronacharya's son with the same name. After killing the elephant, Bhima loudly proclaimed that he had killed "Ashwatthama". Disbelieving him, Drona approached Yudhishthira, knowing of Yudhishthira's firm adherence to Dharma and honesty. When Dronacharya asked for the truth, Yudhishthira responded with the cryptic "Ashwatthama is dead. But the elephant and not your son." Krishna also knew that it would be impossible for Yudhishthira to lie outright. Under his instructions, the other warriors blew trumpets and conchs, raising a tumultuous noise in such a way that Dronacharya only heard that "Ashwatthama was dead", and could not hear the latter part of Yudhishthira's reply. In other versions of the story, it is told that: Drona, in grief, simply does not process the final part of Yudhishthira's statement, or Yudhishthira was simply not loud enough in purpose when he spoke the latter part of his words.