He was named Aswatthaman because he neighed like a horse when he was born. He was also a spark of Rudra - the god of destruction.
In his childhood, he was poor and boys from wealthier families would mix powdered rice with water and claim its milk.
Aswatthaman would dance in happiness thinking he had a taste fresh cow's milk not knowing that he was made a joke of by others...
Later on, his father Drona became the preceptor of the Kuru princes under Bhishma and Kripa's influence and was finally able to provide for his wife and son.
After his father’s death in the war at the hands of Dhrishtadyumna, Aswatthaman vowed revenge. When the war was finally over and Duryodhana was vanquished by Bhima, Aswatthaman along with Kripa and Kritavarman escaped to the woods nearby.
While contemplating at night as to how he should take revenge, he noticed an owl attacking a flock of birds while they were asleep. This inspired him and he set out to infiltrate the Panchalas at night along with Kripa and Kritavarman.
Aswatthaman then kicked Dhrishstadyumna while he was asleep, and he beat him to death with punches and kicks to the throat… The Panchalas along with the Yuva-Pandavas were alerted by the screams they heard and donned their mail for battle.
Many warriors were still confused in consequence of just being woken up from sleep and they beheld Aswatthaman in red, drenched with the blood of his victims, resembling Yama the god of death.
Many tried to escape the carnage but they couldn’t because the exits were blocked by Kritavarman and Kripa, who mercilessly massacred them even though they begged for their life. Some terrified hid under tables and beds but were found by the raging Aswatthaman and were brutally killed in cold blood with sword.
The sons of Draupadi along with Shikandi and others attacked Aswatthaman, but they were all killed by him single-handedly in a brutal fashion.
Shikandi was cut in twain by sword, Pratinvidya was struck in the abdomen, Sutasoma’s arms were chopped-off and he was struck in the flank with sword, Shatanika tried to attack him with a wheel but was beheaded by the furious Aswatthaman, Shrutakarma’s face was smashed with the sword, Shrutakirti was beheaded…
He killed thousands of warriors single-handedly in that awful night and he left the camp by sunrise. He informed Duryodhana, whose thighs had been crushed and who was suffering in agonizing pain. Hearing the news, Duryodhana shed tears of joy and gave up his life-breath.
After the Pandavas were informed of this incident, Bhima set out to kill Ashwatthaman in rage but he knew not that his victim knows the Brahmasira, the most dangerous weapon invented by the Grandsire of the gods - Brahma. Krishna and Arjuna rushed behind Bhima to stop him but by the time they reached, Ashwatthama had already summoned the weapon in a blade of grass. The blade of grass was engulfed in a huge ball of fire and was launched towards Bhima. Immediately Arjuna took an arrow and summoned the Brahmastra and launched it at Aswatthaman. Then sage Narada and Vyasa manifested themselves in the midst of the two warriors and tried to neutralize the two fireballs with their ascetic powers to save the Earth from annihilation.
Arjuna had the ability to withdraw his weapon at the request of the sages but Aswatthaman knew not how to withdraw his, so he focused the weapon towards the wombs of the Pandava women to avoid destroying the world.
Krishna told him that a certain sage had once foretold a prophecy that though the lineage of the Kuru’s would become extinct, only one would survive and for this reason he’d be known as Parikshit. Aswatthaman did not believe Krishna’s words for the weapon can never go fruitless.
Krishna’s cursed Aswatthaman that though the foetus would die, it will be brought back to life and as for him, he shall live for three thousand years suffering in pain, wandering over this earth, without a companion and without being able to talk with anyone. The stench of pus and blood shall emanate from him, and inaccessible forests and dreary moors shall be his abode and that he shall wander over the Earth, with the weight of all diseases…
It is believed by some that he is still alive and some have even claimed to see him, though the curse states only three thousand years, but who knows as per what calculations? Time is calculated in various ways by various people…