Ashwathama (Sanskrit:Aśvatthāmā) or Drauni is the son of guru Drona and the grandson of the sage Bharadwaja. Ashwatthama is a mighty Maharathi who fought on the Kaurava side against the Pandavas in the Kurukshetra War. Ashwatthama is considered as avatar of one of the eleven Rudras and one of the seven Chiranjivis. Along with his maternal uncle Kripa, Ashwatthama is believed to be a living survivor of the Kurukshetra War. The deceptive plot of his rumoured death led to the beheading of his grieving father Drona, who was incapacitated while meditating for his son's soul. Ashwatthama was appointed as the final commander-in-chief of the Kauravas in the Kurukshetra War. Overcome with grief and rage, he slaughters most of the Pandava camp in a single night offensive. Ashwatthama ruled the Northern Panchala being subordinate to the rulers of Hastinapura. According to The Mahabharata, Ashwatthama means "the sacred voice which relates to that of a horse". It is so called because when he was born he cried like a horse.

Drona does many years of severe penance to please Lord Shiva in order to obtain a son who possesses the same valiance as Lord Shiva. He was a Chiranjivi. Ashwatthama is born with a gem on his forehead which gives him power over all living beings lower than humans; it protects him from hunger, thirst, and fatigue. Though an expert in warfare, Dronacharya lives the simple life, with little money or property. As a result, Ashwatthama has a difficult childhood, with his family unable to even afford milk. Wanting to provide a better life for his family, Drona goes to the Panchal Kingdom to seek aid from his former classmate and friend, Drupada. However, Drupada rebukes the friendship, claiming a king and a beggar cannot be friends, humiliating Drona.

After this incident, and seeing the plight of Drona, Kripa invites Drona to Hastinapur. There, he comes upon the attention of his co-disciple Bhishma. Thus, Dronacharya becomes the guru of both Pandavas and Kauravas in Hastinapur. Ashwatthama is trained in the art of warfare along with them.

Since Hastinapura, ruled by King Dhritarashtra, offered Dronacharya the privilege of teaching the Kuru princes, both Drona and Ashwatthama are loyal to Hastinapur and fight for the Kauravas in the Kurukshetra war. Before Drona's death, Ashwatthama visits his father, desiring a blessing of victory that he is denied. Drona advises Ashwatthama to win the war using his own strength and not through a blessing.

On the 14th day of the war, he kills a division of Rakshasas and Anjanaparva (son of Ghatotkacha) and defeats Ghatotkacha using his celestial weapons. He also stands against Arjuna several times, trying to prevent him from reaching Jayadratha, but is eventually defeated by Arjuna.

On the 10th day of the war, after Bhishma falls, Drona is named the supreme commander of the armies. He promises Duryodhana that he will capture Yudhishthira, but then he repeatedly fails to do so. Duryodhana taunts and insults him, which greatly angers Ashwatthama, causing friction between Ashwatthama and Duryodhana. Krishna knows that it was not possible to defeat an armed Drona. So, Krishna suggests to Yudhishthira and the other Pandavas, if Drona were convinced that his son was killed on the battlefield, then his grief would leave him vulnerable to attack. Krishna hatches a plan for Bhima to kill an elephant by the name Ashwatthama while claiming to Drona it was Drona's son who was dead. Ultimately, the gambit works (though the details of it vary depending on the version of the Mahabharata), and Dhristadyumna beheads the grieving sage.

After learning of the deceptive way his father was killed, Ashwatthama becomes filled with wrath and invokes the celestial weapon called the Narayanastra, against the Pandavas. When the weapon is invoked, violent winds begin to blow, peals of thunder are heard, and an arrow appears for every Pandava soldier. This puts fear into the Pandava army, but Krishna, while stopping the troops, advises that the army lay down all its weapons and surrender to the weapon. As himself being the incarnation of Narayana, he knows about the weapon, as the weapon only targets an armed person while ignores unarmed ones. After getting their soldiers to disarm (including Bhima with some difficulty), the astra passes by harmlessly. Narayanastra failed to harm Arjuna and Krishna as they both were divine persons (Krishna himself is Narayana & Arjuna is Nara). When urged by Duryodhana to use the weapon again, desirous of victory, Ashwatthama sadly responds that if the weapon is used again, it will turn on its user. After the use of Narayanastra, a terrible war between both armies takes place. Ashwatthama defeats Dhrishtadyumna in direct combat, but failed to kill him as Satyaki and Bhima cover his retreat. As the battle continues, Ashwatthama manages to kill King Nila of Mahishmati.

After the terrible death of Dushasana, Ashwatthama suggests Duryodhana to make peace with the Pandavas, keeping in mind the welfare of Hastinapur. Later, after Duryodhana is struck down by Bhima and facing death, the last three survivors from the Kaurava side, Ashwatthama, Kripa, and Kritvarma rush to his side. Ashwatthama swears to bring Duryodhana revenge, and Duryodhana appoints him as the commander-in-chief.

Along with Kripa and Kritavarma, Ashwatthama plans to attack the Pandavas camp at night.  Ashwatthama first kicks and awakens Dhrishtadyumna, the commander of the Pandava army and the killer of his father. Ashwatthama strangles the half-awake Dhrishtadyumna by choking him to death as the prince begs to be allowed to die with a sword in his hand. Ashwatthama proceeds with butchering the remaining warriors, including Shikhandi, Yudhamanyu, Uttamaujas, and many other prominent warriors of the Pandava army; many texts put an emphasis on his exploits against those from Panchal. He also kills Draupadi’s children in their sleep. Even as some soldiers try and fight back, Ashwatthama remains unharmed due his activated abilities as one of the eleven Rudras. Those who try to flee from Ashwatthama's wrath are hacked down by Kripacharya and Kritavarma at the camp's entrances. After the slaughter, the three warriors go to find Duryodhana. Duryodhana dies in front of them.

The Pandavas and Krishna who were away during night, now return to their camp the next day morning. Hearing the news of these events Yudhishthira faints and the Pandavas become inconsolable. Bhima angrily rushes to kill Drona's son. The Pandavas, along with Krishna, went to save Bhima. They found him to sage Vyasa's ashram near the bank of Bhagirathi. The now triggered Ashwatthama invokes the Brahmashirastra against the Pandavas to fulfill the oath of killing them. Krishna asks Arjuna to fire the Brahmashirastra against Ashwatthama to defend themselves. Vyasa intervenes and prevents the weapons from clashing against each other. He asks both Arjuna and Ashwatthama to take their weapons back. Arjuna, knowing how to do so takes it back. Out of rage, Ashwatthama instead directs the weapon towards the womb of the pregnant Uttara in an attempt to end the lineage of the Pandavas. The angered Pandavas want to kill Ashwatthama, but Sage Vyasa stopped them to do this. As a punishment, Ashwatthama was asked by Vyasa to surrender the gem on his forehead to Pandavas. Krishna then curses Ashwatthama that he will roam in the forests with blood and puss oozing out of his injuries and cry for death for 3000 years. Since he had no fear of death during war, death would not meet him.

On the 12th day of war Ashwatthama kills Nila

Beholding Prativindhya staying at the van of that dreadful battle, Drona's son (Ashwatthaman), desirous of protecting the honour of his sire, resisted the former with his shafts. Prativindhya, then, excited with rage pierced Ashwatthaman, bearing on his standard the device of a lion's tail and staying in battle for the sake of his father, with many sharp shafts. The (eldest) son of Draupadi then scattered over Drona's son showers of arrows, like a sower scattering seeds on the soil at the sowing season.

Then Nila, resembling a veritable fire, his arrows constituting its sparks and his bow its flame, began to consume the Kuru ranks, like a conflagration consuming heaps of dry grass. The valiant son of Drona, who from before had been desirous of an encounter with him, smilingly addressed Nila as the latter came consuming the troops, and said unto him these polite words,

“O Nila, what do you gain by consuming so many common soldiers with your arrowy flames? Fight with my unaided self, and filled with rage, strike me.”

Thus addressed, Nila, the brightness of whose face resembled the splendour of a full-blown lotus, pierced Ashwatthaman, whose body resembled an assemblage of lotuses and whose eyes were like lotus-petals with his shafts. Deeply and suddenly pierced by Nila, Drona's son with three broad-headed arrows, cut off his antagonist's bow and standard and umbrella. Quickly jumping down from his car, Nila, then, with a shield and an excellent sword, desired to sever from Ashwatthaman's trunk his head like a bird (bearing away its prey in its talons). Drona's son, however, by means of a bearded arrow, cut off, from his antagonist's trunk, his head graced with a beautiful nose and decked with excellent ear-rings, and which rested on elevated shoulders. That hero, then, the brightness of whose face resembled the splendour of the full moon and whose eyes were like lotus-petals, whose stature was tall, and complexion like that of the lotus, thus slain, fell down on the earth. The Pandava host then, filled with great grief, began to tremble, when the Preceptor's son thus slew Nila of blazing energy.