On the 16th Day of the Mahabharata War, Karna took up the leadership of the Kaurava Army.
The previous day, their former commander-in-chief, the invincible Dronacharya, had been struck down by cunning and deceit. Bheema had killed an elephant named Ashwatthama and proclaimed his achievement to Drona, that "Ashwatthama was dead". Drona, mistaking it for his own son, went to confirm this with Yudhistira, who was known for his truthfulness.
Knowing that there was no other way to defeat the mighty Dronacharya, Yudhistira lied that this was indeed true. Losing the will to live, Dronacharya discarded his weapons and sat down in meditation. Seizing this opportunity, Drishtadyumna, the fire-born prince of the Panchala kingdom, beheaded Dronacharya, ending his unstoppable rampage.
Karna knew that the Kaurava forces were severely demotivated after this. He rallied the army under his banner in the form of the Makaravyuha (crocodile formation), with himself at the snout, Shakuni and Bhagadatta at the eyes, Ashwatthama at the crown of the head, Dusshasana at the neck and Duryodhana at it’s heart. The forelegs were formed by Kritavarma and Kripacharya, the hindlegs by Susharman and Shalya.
The Pandavas on the other side, formed the Chandrakalavyuha (pale crescent moon), which was very thin indeed, owing to their now depleted forces. Bheema lead the left tip, Drishtadyumna led the right tip and at the heart of the vyuha was Arjuna with Yudhistira, Nakula and Sahadeva.
The battlefield itself was littered with cracks and craters after the impact of various celestial weapons.
Map of the Battlefield