Dronacarya was killed on the battlefield on the 15th day. Dhristadyumna jumped on the chariot, grabbed him by the dreadlocks and cut his head right off. He grabbed his severed head by the thick dreadlocks spun it around and slam-dunked it right there on the ground of the battlefield. When Dhristadyumna did that to Drona who was so strong, all the Kurus who saw it ran. They just started running for there lives.
Aswathama started walking towards Duryodhana because he wanted to know what had happend. He wanted to know why everyone was running because his father was the general and if everyone was running than that meant that something bad had happend. When he met Duryodhana, Duryodhana couldnt even look him in the eye.
Now I have to explain something about Aswathama. He was very vicious. He was very tall. He was tall like Bhima tall. Very thick bodied. He was a Brahmin but he was like a bull. They said in three ways he was like a bull: His deep voice, His tread, and his neck. He wore red because he was a Brahmachari which was what Bhramacharis wore at that time. He covered his body with red sandal paste, had three tilak lines of red on his forehead and massive dreadlocks on his head. His eyes glowed from his potency. People didn't even want his shadow to touch them.
So this grim figure was standing there before Duryodhana and demanded “What happend.” The great king of Hastinipura could not even look at him so he said “Kripacarya, you tell him.” So Kripacarya was his uncle because Drona had married Kripi who was the sister of Kripa. Kripa said, “They tricked Drona, they told him that you were killed, so he decided to quit the battle and seeing your father in that depressed state Dhristadyumna leapt upon his chariot and cut his head off, so now the whole army has run away."
Aswathama started roaring and the whole fleeing army stopped because his roar was so loud. He jumped off his chariot and started pounding his feet on the ground. He started leaping and dancing like a mad man roaring louder and louder like a lion. All the warriors immediatly stopped running because they realized that here is this great hero who has so much mystic power and who has been enraged by the death of his father.
Raising his bow and standing before the assembled heroes Aswathama spoke with a voice like thunder, “My father has given me a very special weapon in case of emergency called the Narayanastra and this weapon has the potency as Narayan. If I unleash this upon the enemy then they are all destroyed.” To a chorus of raised cheers Aswathama chanted the mantras for the weapon and unleashed it at the Pandavas.
All of a sudden big black clouds formed on the battlefield which began raining hot coals and severe winds began to swirl. A giant wave of fire formed on the horizon which sped at the Pandava host like a giant tsunami of fiery doom.
All the generals came before Krishna and asked him what had happened.
Krishna said “Aswathama has used the Narayanastra which will certainly kill us all. However I know a secret to this weapon which will nullify it. We must all cast aside our weapons and armor and lay on the ground prone. In this way the weapon will pass right over us.”
The whole army began to dismount and discard their arms and armor. However one person stood firm at the head of the army dressed for war; Bhima.
Bhima stood at the head of the army with his club on his shoulder.
Arjuna came forward and said “Hey! Hey Bhima get down! Take off your armor and throw down your club!”
Bhima looked out across the battlefield at this tidal wave of flaming weapons and then looked back at Arjuna and said. “Look, I will tell you three reasons I am not going to do that. The first is that this weapon is called Narayanastra and it has the same potency as Narayana. So I am not worried because if it kills me I will get moksa. The second reason is that Krishna is my friend. Here is Krishna.” He pointed to Krishna who was broadly grinning the whole time. “In case you did not know, he is all powerful and the cause of all causes. So I have no fear because he is my friend and he will not let me be hurt.” He had such firm faith. “The main concern is, you see Duryodhana is standing over there and I’ll be damned if I am going to lay down on the ground like some coward in front of him. So younger brother I am afraid I cannot do that.”
So this terrible weapon started speeding at him. This tsunami of flaming weapons came racing at Bhima, its total power concentrated on him alone. And he is just staring it down with the club on his shoulder and his arms crossed. His total faith that no harm would come to him. The flames started to singe his eyebrows and he snorted "Hummf" then all of a sudden it was gone!
You see the thing is, his faith in the protection of Supreme Lord was so strong that it couldn’t touch him.
Anyone who hears this story will also gain complete faith in the Supreme Personality of Godhead Krishna.