The story of killing of the sons of Draupadi by Ashwatthama, safeguarding the life of Parikshit in the womb of Uttara

At the end of the battle of Mahabharata Duryodhan was about to die after being hit by the mace of Bhima when Ashwatthama (son of Guru Dronacharya) came to him with the bodiless heads of five sons of Draupadi to show his bravery. After the defeat of the Kauravas, Ashwatthama wanted to please Duryodhan. So he beheaded the five sons of the Pandavas stealthily when they were asleep. The despicable deed of Ashwatthama devastated Draupadi. Seeing the grief-stricken Draupadi, Arjun promised her to kill Ashwatthama as revenge and asked his dearest friend and guide Lord Krishna to accompany him as his charioteer in the quest for the assassin. As such Ashwatthama was feeling dejected committing such a heinous crime when he saw Arjun coming after him. He tried to run away but failed. Finding himself no match to Arjun he drew the Brahmastra to attack. Brahmastra was a very powerful weapon that could be received only by performing severe penance. It can be achieved by many at a time subject to the said penance. Brahmastra remains invisible and it can be evoked only after chanting some hymn. When Arjun saw the weapon coming towards him he asked Lord Krishna the procedure to retaliate Brahmastra and the Lord advised him to cast his own Brahmastra. Arjun was also a holder of Brahmastra and he retaliated with the same. So the power of both the weapons got neutralised only after creating havoc in the locality. Now Arjun caught hold of Ashwatthama but couldn't kill him because he was a Brahmin and the son of his fight master (Guru) Dronacharya in spite of Lord Krishna's advice to finish him. Actually, the Lord was testing the conscience of Arjun. So Arjun chained him with his chariot and took him to Draupadi to fix the punishment. But Draupadi being a kindhearted and conscientious lady took pity on Ashwatthama and advise Arjun not to kill the son of his Guru and let him go. But Bhima objected to it. So to teach a lesson for the crime Arjun pulled out the gem attached to the forehead of Ashwatthama and also made him baldheaded to humiliate in eye of the public. At that time Ashwatthama left the spot silently, but silently he promised to himself to take revenge for his humiliation and decided to kill the child of  Uttara, wife of Abhimanyu and grand daughter in law of Arjun. This child was the only successor and heir to inherit the throne of Hastinapur after the Pandavas. So after some days, Ashwatthama attacked the womb of Uttara with Brahmastra to kill the child. Seeing the weapon coming towards her Uttara ran to Lord Krishna and prayed him to save her womb. The Lord neutralised the weapon with his divine power and saved the heir of Pandavas. This child was named as Parikshit who became the emperor of Bharat after Yudhisthir relinquished the throne. Then the went to the battlefield where the great warrior Bhishma lying on the bed of arrows (शरशैया). He liberated Bhishma who was waiting for the Lord to take his last breath infront of him. Thereafter Lord Krishna left the Pandavas and went to his Kingdom Dwarka.