At a critical moment in the Mahabharatha, Arjuna, the great warrior, is tired of fighting. He has reached the point where he questions the value of fighting, the purpose of it, and whether or not he has the strength the continue. This doubt is almost paralyzing for Arjuna as fighting is a challenge, but fighting against his family is almost an insurmountable entity for him. His doubt, his questioning, his reticence to continue represent the moment where Krishna, as the universal lord Vishvarupa, appears and guides Arjuna. Krishna, being Arjuna's charioteer, rises to the heavens and answers Arjuna's query. Krishna reminds Arjuna that what needs to be done is not something that has to be examined for its own reward, but rather should be done because it is one's duty. The fruit of the labor holds no importance as opposed to the labor itself. Arjuna's dharma, his duty, is something that has to be done. To prove this, Krishna manifests himself as the ultimate essence of being as the Vishvarupa. Arjuna beholds the vision of Krishna being everything at once:
Having spoken thus, Lord Shri Krishna, the Almighty Prince of Wisdom, demonstrated to Arjuna the Supreme Form of the Great God, the Universal Form, Vishvarupa. The colossal and unimaginable form of the Lord was a true vision, comprising countless eyes and mouths and mystic forms innumerable, with glistening ornaments and blazing celestial weapons. The visionary form was crowned with divine garlands, clothed in shining garments, anointed with divine unction; Krishna demonstrated Himself as the Resplendent One, Marvellous, Unbounded and Omnipresent. "Could a thousand suns blaze forth together it would be but a faint reflection of the radiance of the Lord-God." In that lifetime vision Arjuna witnessed the universe, with its manifold shapes, all encompassed within One, its Supreme Lord. The universal form of Krishna, Vishvarupa was without beginning, without middle and without an end; He was infinite in His prowess, His arms all-embracing like the sun and moon and His eyes, His face was as if radiating with the fire of sacrifice, inundating the whole universe with light. Krishna alone had filled all the quarters of the sky, earth and heaven and the regions within.
At this moment, Arjuna understands his dharma, his purpose as he has beheld something that few others have and comprehends his purpose in an "other worldly" manner. This infinite level of comprehension allows Arjuna to see things as they truly are. Krishna appears as the Visvarupa once and this becomes enough to drive Arjuna, and Krishna's conception of justice and ethics, to victory.
Vishwaroop refers to the the ultimate God appearing in a forms that incorporates the complete creation or Universe in it. The word Vishwaroop is formed by joining two Sanskrit words, vishwa meaning the universe and roop meaning form. As per Hindu philosophy, this wishwaroop is not just a visual appearance, but a reality.
The God of Hindu philosophy, called bhagwan, is not like the roman or other similar gods. Counterpart of such gods in Hindu mythology are known as devas, and by one account there are supposed to be 330 million such devas, but there is only one ultimate God called Brahma, Bhagwan and many other names. This god is not just the creator of the universe but also its cause and effect. A rather complicated concept, about which many great scholars also get confused. Most certainly, I understand very little of it. My little explanation here is only intended to illustrate the complexity of this concept.
Lord Krishna is an Incarnation in human form of this God. There is a incident in Mahabharata in which Arjuna, a friend of Lord Krishna, becomes confused about his duty just before start of the great battler of Mahabharata between Arjun and his four brothers called Pandavas, and their one hundred first cousins called Kauravas, and tells Lord Krishna that he will not fight his own cousins and other kins. The advise given by Lord Krishna to Arjuna in response is the subject matter of the famous Hindu religious text called Bhagwad Geeta or just Geeta.
Lord Krishna reveals Vishwaroop to Arjun during the course of this discourse between them described in Geeta. To be exact this Chapter 11 of Geeta records the detail of how this event, called Vishwaroop Darshan, actually took place. To be able to see the Vishwaroop one needs to have some kind of super vision called divya drishti. Lord Krishna bestows divya drishti upon Arjuna temporarily. Another person, Sanjay was also endowed with divya drishti for the period of the battle of Mahabharata. Therefore, Arjuna and Sanjay were the only two persons who actually witnessed the Vishwaroop. This is the only occasion in whole of Mahabharata when Lord Krishna revealed the Vishwaroop to anyone, and to my knowledge there is no other mention of Vishwaroop Darshan in any of the ancient texts.
1. To Duryodhana, when Lord Sri Krishna goes as the messenger
a. This is because Duryodhana asked for ‘veer swarga’ to Lord Sri Krishna
2. To Vidhura – when Lord Sri Krishna visits him at his house
3. To Arjuna – when he was depressed to fight against his own family and kin
4. To Bhishma – on his death bed lying upon a bed of arrows
5. To Drithrashtra (briefly giving him vision to see HIM in full)
6. To Karna – just before he was killed, Lord Sri Krishna accepts all the punya accumulated by Karna