Parva 10
Sauptika Parva
The war was won. That night, three men walked back toward the camp
Three in the forest
Only three men on the Kaurava side could still fight, Ashwatthama, Kripacharya, and Kritavarma. The three of them sat down together in a forest at night. Duryodhana was still alive, lying on the field with his shattered thigh.
The owl and the crows
Ashwatthama could not sleep. He looked up. Crows were roosting in a tree. Then an owl arrived and killed the sleeping crows one by one. Ashwatthama watched. Then he woke his companions and said he had found a way.
Kripacharya's refusal
Kripacharya tried to stop him. He said killing sleeping men was not the work of a kshatriya, that it would bring no honor, and that night had already fallen, better to wait for morning. Ashwatthama would not listen. He said his own father had not been killed by the rules either. The three set out toward the camp.
The gate of the camp
At the gate of the camp stood a huge, terrifying figure that swallowed every weapon Ashwatthama threw at it. He understood the door would not open by force. He built an altar there, lit a fire, and offered himself as the sacrifice. Then Mahadeva appeared. He said the Panchalas had survived this long only because Krishna was protecting them, and that protection had now run out. He gave Ashwatthama a sword, and entered his body himself. The gate opened. The text tells it this way deliberately, that what happened that night was not Ashwatthama's alone.
The killing of Dhrishtadyumna
Inside, everyone was asleep. Ashwatthama first sought out Dhrishtadyumna, the man who had beheaded his father. He woke him, threw him to the ground, and beat him to death with his fists and feet. Dhrishtadyumna asked for a sword, to die like a warrior, Ashwatthama refused him one. Then he moved through the rest of the camp. Draupadi's five sons were killed that night, Shikhandi was killed, and everyone else still left. Outside, Kripacharya and Kritavarma stood at the gates, and killed anyone who fled past them, then set the camp on fire. An entire army was wiped out in one night. None of the five Pandavas were in the camp that night, they were staying outside with Krishna.
The jewel and the Brahmashira
When it was discovered in the morning, Draupadi sat down on the ground. She said she would not rise until she was brought the gem set in Ashwatthama's skull, which had been part of him since birth. The Pandavas went after him. Ashwatthama, surrounded, released the Brahmashira weapon. Arjuna released the same weapon in answer. Vyasa and Narada came between them and told both to withdraw the weapons, or nothing would remain. Arjuna withdrew his. Ashwatthama could not, he had never learned how. So instead he turned it toward the wombs of the Pandava women. The child in Uttara's womb was struck by that same weapon. Krishna made a vow on the spot, that the child would be born dead, but he himself would bring it back to life. The gem was pulled from Ashwatthama's head, and he was cursed, to wander for three thousand years, carrying his wounds, alone.
Note: The owl scene, Kripacharya's refusal, Shiva opening the gate, the killing of Dhrishtadyumna, the killing of Draupadi's five sons, the jewel, the Brahmashira and Krishna's word are all in the Sauptika Parva of Vyasa's Mahabharata. But Ashwatthama's immortality — that he still wanders today — comes from later religious tradition. In the text the curse is for three thousand years, not for immortality. This book is the darkest part of the whole story, and the text makes no attempt to soften it.
Source: Mahabharata (ascribed by tradition to Vyasa), Sauptika Parva