Dronacharya’s son, Ashwatthama was born with a gem stuck to his forehead. The stone was what gave him power over all living beings that were lower than humans. It also protected him from fatigue, thirst and hunger.
Ashwatthama fights on the side of the Kauravas in the Kurukshetra war and is infuriated when Yudhishthira uses his name to disarm Drona and kill him. In a fit of rage, Ashwatthama fires up the Narayanastra, a divine weapon that could have killed all the Pandavas and the Pandava army. However, Krishna, on realising what had been fired orders everyone to put down their weapon. Narayanastra was a great, powerful weapon but it didn’t attack anyone that wasn’t armed. And so when everyone puts down their weapons, the Narayanastra passes over without harming them.
As the war ends – effectively it ended when Duryodhana was fatally injured in a face-to-face combat with Bhima – Ashwatthama, his maternal grandfather Kripa, and Kritvarma attack the Pandava camp and kill several soldiers, warriors and princes in their sleep. They also kill Pandavas’ young sons. Ashwatthama reports back to Duryodhana that his victors won’t be celebrating with their son. Hearing this news, Duryodhana breathes his last.
The Pandavas are furious and track down Ashwatthama at the hermitage of Vyasa. As Arjuna and Ashwatthama face off each other for the first time, the latter draws the dreaded Bramhastra, a weapon that had the reputation of wiping out everything that stood in its path. As Arjuna fires the Bramhashira, a counter missile to defend themselves, all hell breaks loose.
Knowing the potential for damage with these two weapons, Vyasa steps in and stops the two in their path. While Arjuna takes back his weapon, Ashwatthama is unable to do so and therefore directs it to the womb of Uttara, Arjuna’s wife who had been carrying their child at the time. Krishna rushes to her and saves the child from being killed and returns to pluck the gem out of Ashwatthama’s head.
He then curses Ashwatthama that for his dark deeds, he will suffer till the end of the kalyuga and never die. And so it is that Ashwatthama comes to be an immortal, but one who suffers and goes around begging for oil to heal his wounds.