Krishna in his Vishwaroop

Vishwaroop is the eternal, all encompassing magnificently glorious form of Vishnu into whom everything assimilates and from whom everything in this Universe is sprung. Events which led to Krishna's Vishwaroop are listed as under

1. Yasoda: Bal-Krishna inspired Gokul with many leelas. Not bereft of childish tendencies, his love for fresh butter was insatiable. Many a times, he with his companions, would secretly feast on butter. Yashodha had to deal with several complains for this nature of Krishna and one day she wanted to establish that he had been secretly eating butter. She asked bal-Krishna to open his mouth. When she peeked inside she saw the entire universe in his mouth and was awestruck. Krishna revealed his Vishwaroop to Yashodha and comforted her.
2. Akroora: as Kamsa's emissary Akroor came to invite Krishna and Balaram to Mathura for a  decisive due with Kamsa (Krishna's evil maternal uncle who wanted to put Krishna's life to an end after a premonition that declared Krishna to be the cause of Kamsa's death). At this time Akroor requests Krishna to appear in his Vishwaroop
3. Duryodhan: Krishna visited Hastinapura as the 'Shanti-Doot' (messenger) of Pandus requesting back their Indraprastha (kingdom) after the 13 years of 'Vanvas' and 1 year of 'AgyatVas'. Duryodhan blinded by his rage did not want to spare land even as small as the tip of a needle, instead he wanted to keep Krishna as a prisoner. At his point Krishna assumed his Vishwaroop

4. Arjuna, Hanuman (as his image atop Arjun's chariot) and Sanjaya (who was relating the battle events to blind king Dhritarashtra): in the battle of Kurushetra while reciting Bhagvad Gita, Krishna assumed his Vishwaroop to convince Arjun to take up his responsibilities as a warrior, defeat the Kauravas to re-establish 'darma' on this world
5. Karna: He was son of Surya (Sun God) born as a boon to Kunti before her marriage and hence was abandoned. Years passed and Karna, evolved as a great warrior, was befriended by the evil Duryodhan. As a prelude to the battle, Kunti requested Karna

   - To not inflict fatal wounds on Pandavas,
  - Not use more than once any of his celestial weapons (received as boon from his guru Parshuram)
Karna was a great warrior and had been truthful to all his vows, was 'Danveer' (helped those who asked him for anything) even in his death-bed and gave away his 'Punya' (positive karmic results). In return for his virtues, Krishna appeared in his Vishwaroop when Karna was breathing his last
6. Devavratha a.k.a Bhishma (thus named for his tremendous

'bhishma pratigya' to remain ever celibate): God-son of 'Ganga', Bhishma had earned many a boons. One among them was his ability to control his time of death. This rendered him as the greatest impedance to the Pandavas' victory in battle. However, among many 'leela' of Krishna, one was the means to bring down Bhishma in battle. Lay on a bed of body piercing arrows shot by Arjuna, in dying breath, Bhishma requested Krishna to appear in his Vishvaroop to bless his mortal life. For his many a great deeds and upright life, Krishna obliged.
7. Utanga: A childhood Brahmin friend of Krishna, Utanga, was unaware of the turn of events and Kurushetra battle. After the battle, on his way back, Krishna met Utanga. Later wanted to know about the well being of the Pandavas and Kauravas. Krishna narrated the events, great loss of lives and war between brothers. Upon hearing this, Utanga lost himself in fury. To pacify him, Krishna appeared in his Vishwaroop.