There were now just three Kauravas left and foremost amongst them was Ashwatthama, the son that Guru Dronacharya thought he had lost to the enemy. As was expected he was thirsting for revenge.
So one night when he knew that everyone in the enemy camp must be asleep, he took hold of his weapons and approached the entrance. Here he was met by a great being who was spitting fire. He tried to destroy this being but just couldn't succeed as this creature swallowed whatever was being thrown at him. He then remembered everything he had been taught and the folly of his ways and retreated.
He sat down to pray to Lord Shiva for help. He sang his praises and prayed with all his heart until a golden altar appeared in the sky. This was accompanied by a host of fearful creatures that normally accompanied Lord Shiva, the god of destruction. They knew that kind-hearted Shiva was going to respond to his prayers. And that they would have a feast after the forthcoming slaughter.
But in truth Shiva had not yet responded to Ashwatthama's prayers and the sacrificial altar was still awaiting a live sacrifice. Now there was just one thing to do and so he decided to sacrifice himself. He prayed to Shiva saying that as he could not defeat his enemies alone he was coming to the lord.
He then began his ascent towards the altar and took a step towards the fire. And suddenly Lord Shiva appeared before him saying that he dearly loved Lord Krishna and had therefore cast his protective hand over the Pandavan army and ensured their victory. But now he would protect them no more and their time on earth was over. Saying this the Lord entered Ashwatthama's body and handed him a shining silver sword.
Fortunately the five Pandavas, Lord Krishna and another man were missing from the camp that fateful night and were saved from the ensuing slaughter.
Ashwatthama marched in followed by Shiva's faithful followers and wreaked havoc in the camp. Not a soul was allowed to escape and not a person was left alive.
The next morning the Pandavas were shocked to hear the news and asked Krishna how it was possible for Ashwatthama and his two companions to destroy a whole army. Lord Krishna explained that the great Lord Shiva himself, who was not only easily offended but easily pleased as well, had aided him. Which is what the smart Ashwatthama had done through his prayers to secure the great god's favour.
In this way Ashwatthama was able to avenge his esteemed father, Dronacharya's death.