Bhagadatta was the King of Pragjyotishpura(present day Assam). He was the son of the demon king Narakasura who was killed by Krishna. In the Mahabharatha war, Bhagadatta joined the Kaurava forces against the Pandavas. He used to fight sitting atop a huge war elephant who was like a mountain itself. During the war, Bhagadatta & his elephant terrified the Pandava army and ultimately Krishna had to bring Arjuna to the battle scene to save the Pandava army from the deadly combination of Bhagadatta and his elephant.
Arjuna immediately stuck Bhagadatta with a shower of sharp arrows which caused him excessive pain. Bhagadatta was filled with rage and invoked the Vaishnava weapon by chanting mantras and hurled it at Arjuna . Now Krishna, covered Arjuna and received the weapon on his chest instead, at which point the weapon turned into the Vaijayanti garland. Arjuna's pride was hurt and he was not pleased. He said to Kesava, "You had pledged to only be my charioteer during the war and I am supposed to fight against the enemies and counter their weapons. As long as I'm standing, you are not supposed to fight. With my Gandiva, I am capable of fighting against all the gods, demons & humans. Why did you then not allow me to counter this weapon".
Vasudeva smilingly said to Arjuna, listen to this secret and ancient history. I have four forms, which are eternally engaged in protecting the worlds. Dividing my own Self, I ordain the good of the worlds. One form of mine, staying on the earth, is engaged in the practice of ascetic austerities. Another beholds the good and the evil deeds in the world. My third form, coming into the world of men, is engaged in action. My fourth form lies down in sleep for a thousand years. This form after awakening grants excellent boons to persons deserving of them. The earth, knowing this on one occasion had come and asked me to grant the Vaishnava weapon to her son Naraka. The Vaishnava weapon made Narakasura invincible and enabled him to conquer all the devas and become king of the demons. When I killed Narakasura, his son Bhagadatta got this weapon. There is no one in the world, including even Indra & Rudra who can stand up against his weapon of mine. It was to save you, that I had to break my vow and baffle it myself. Now, you can fight with Bhagadatta and kill that enemy of Pandavas.
Arjuna, rejoiced after hearing this story first killed Bhagadatta's elephant and then Bhagadatta himself.
