After the fall of Bhishma, the Kauravas felt great void in their heart, and felt that their strength would be inadequate to face and defeat Arjuna and Sri Krishna combine unless Karna takes up the reigns of leading the Army. However, when approached, Karna declined to take the responsibility saying, "Guru Drona will be the ideal choice as the Chief Leader for Kaurava Army." Thus from 11th to 15th day Acharya Drona took the command in his hand and caused great destruction to Pandava Army. Sri Krishna and Arjuna became nervous and were worried about the outcome. Drona was a great warrior with supreme expertise in the game of war and all the instruments of war. It was not possible for anyone to defeat him.
Meanwhile Sri Krishna reminded Bhima about his powerful son Ghatotkacha born of demon wife Hidimbi! Ghatotkacha had special supernatural powers of getting invisible, smaller or larger in size, etc. at will. Sri Krishna suggested Pandavas to call this son of Bhima who would wreck havoc in Kaurava army and Karna would be forced to use his Special Vaasavi Astra on him instead of Arjuna.
Accordingly Bhima wished Ghatotkacha to come to Kurukshetra and indeed, as soon as his father wished, as promised by him, this great and powerful warrior was there in front of Bhima and Sri Krishna. He was told to fight and annihilate the Kauravas army next day to which he gladly agreed. Next day the battle began. Karna was in search of Arjuna, but Ghatotkacha obstructed his path. Moreover, Ghatotkacha vomited fire and threw large quantity of boulders and sand on Kauravas. There was total chaos in Kauravas camp. Soldiers were running here and there. Scores of their army men were killed.
Duryodhana now requested Karna to kill this demon otherwise, he said, "Nobody will be alive." Reluctantly Karna had to use 'his one time use' Vaasavi Astra on Ghatotkacha instead of Arjuna. Thus, Ghatotkacha died but saved the life of Arjuna.
Death of Abhimanyu
The death of many stalwarts put Duryodhana in a fix. Next day he arranged his army in an impregnable wheel format known as "Chakra Formation". On the side of Pandavas only Abhimanyu, son of Arjuna was capable of penetrating this format; but he did not know how to come out of it. This teenage lad, however, pleaded with his father and uncles to allow him to enter the enemy ranks and then all of Pandavas would come to help him inside.
Accordingly, seeking blessings of Sri Krishna, Abhimanyu led small army and entered the Chakra formation. Kauravas allowed him to enter but soon sealed off the entry of Arjuna and company. Duryodhana, Dushasana, Jayadratha, and many more Kauravas warriors attacked the lone but brave Abhimanyu who fought gallantly all alone against them all.