Duryodhan came to shackle and imprison Krishna when he had come to Hastinapur with peace treatly , what exactly happened at that time ? Did Krishna show his Vishwa-Roop to everyone there or did he create an illusion to confuse Duryodhan ?
It was a divine form but not the one from Bhagwat Gita.
In case of hastinapura sabha, Krishna displayed a divine form, the form that he displayed was a sum total of celestial ‘support’ that was with him. Krishna says to Duryodhana how foolish was the latter’s understanding about Krishna that the latter considered Krishna to be alone and designed plans to make him captive, Krishna’s divine form was a response to Duryodhana’s foolishness where he showed that it was impossible for Duryodhana to succeed in his motives.
”vaiśampāyana uvāca vidureṇaivamuktastu keśavaḥ śatrupūgahā। duryodhanaṃ dhārtarāṣṭramabhyabhāṣata vīryavān॥ 1 eko’hamiti yanmohānmanyase māṃ suyodhana। paribhūya sudurbuddhe grahītuṃ māṃ cikīrṣasi॥ 2 ihaiva pāṇḍavāḥ sarve tathaivāndhakavṛṣṇayaḥ। ihādityāśca rudrāśca vasavaśca maharṣibhiḥ॥ 3 evamuktvā jahāsoccaiḥ keśavaḥ paravīrahā॥“ (MBH 05:131:1-2).
“Vaisampayana said, ‘After Vidura had said this, Kesava, that slayer of hostile divisions, endued with great energy, addressed Dhritarashtra’s son, Duryodhana, and said, ‘From delusion, O Suyodhana, thou regardest me to be alone, and it is for this, O thou of little understanding, that thou seekest to make me a captive after vanquishing me by violence. Here, however, are all the Pandavas and all the Vrishnis and Andhakas. Here are all the Adityas, the Rudras, and the Vasus, with all the great Rishis. Saying this Kesava, that slayer of hostile heroes burst out into a loud laughter”.
Then in his divine form are seen, five pandavas surrounding him from all sides, and protecting him with upraised arms. There were rudras, adityas, vasus surrounding Krishna in all sides. On his forehead appeared Brahma, on his chest appeared Rudra and from his other parts other gods issued forth. There appeared his brother Balarama standing by his left side wielding his plough, Arjuna manifested with his bow stretched standing by his right side. Behind Krishna were seen standing Bhima, Yudhishthira, Nakula and Sahadeva. In front of Krishna were seen andhakas, vrishnis, headed by Krishna’s son Pradyumna all with their upraised arms and ready to strike. And Krishna was seen in his native Vishnu form holding discus, conch, mace, bow, nandakam (Vishnu’s sword), and many other weapons. From his eyes, and pores of his body emanated fierce sparks of fire mixed with smoke.
One may refer to the verses 3-13 from chapter 131 of Udyoga Parva from Mahabharata for these details. I’m skipping quoting them because of the length of those verses and translation. The summary has already been provided, which should suffice.
People may easily consider this as the same Vishvarupa seen by Arjuna, but it is not so. We need to carefully read the further verses here. This divine form was doubtlessly great but didn’t require divine eyes to be seen. Everyone was able to see that awful form by their eyes of flesh. However the sinful people got terrified seeing that form and closed their eyes. For the pious hearted people such as Bhishma, Drona, Sanjaya, Vidura and the various ascetics and sages who were present there – Krishna had bestowed a divine sight because of that divine sight they were able to behold him without getting scared.
“taṃ dṛṣṭvā ghoramātmānaṃ keśavasya mahātmanaḥ॥ 13 nyamīlayanta netrāṇi rājānastrastacetasaḥ। ṛte droṇaṃ ca bhīṣmaṃ ca viduraṃ ca mahāmatim॥ 14 sañjayaṃ ca mahābhāgamṛṣīṃścaiva tapodhanān। prādātteṣāṃ sa bhagavāndivyaṃ cakṣurjanārdanaḥ॥” (MBH 05:131:13b-15).
“And beholding that awful form of the high-souled Kesava, all the kings closed their eyes with affrighted hearts, except Drona, and Bhishma, and Vidura, endued with great intelligence, greatly blessed Sanjaya, and the Rishis, possessed of wealth of asceticism, for the divine Janardana gave unto them this divine sight on the occasion”.
This clearly shows that the divine form shown in the kuru sabha, could be seen by everyone without needing any divine eyes, however, without divine sight that form was scary to look at, and for the good people Krishna gave the divine sight which helped them appreciate the grandeur of that form instead of getting frightened.
But in Bhagawad Gita. Arjuna asked Krishna to display the cosmic form. Krishna however said to Arjuna that with his normal eyes of flesh he wouldn’t be able to see that cosmic form and gave him divine eyes as stated in below verse.
“na tu māṃ śakyase draṣṭumanenaiva svacakṣuṣā। divyaṃ dadāmi te cakṣuḥ paśya me yogamaiśvaram॥“ (MBH 6:35:08).
“Thou art, however, not competent to behold me with this eye of thine. I give thee celestial sight. Behold my sovereign mystic nature’”.
This is unlike the kuru sabha, where everyone was able to see the form with normal eyes, whereas divine sight was given only to eradicate fear. Here however, only after getting the divine eyes, Arjuna was able to see that cosmic form.
Here another noteworthy point is that – despite having received divine sight, Arjuna, the brave warrior who even crushed the army of demons called ‘nirvatkavachas’ with his might, who wrestled with Mahadeva and pleased him – such a brave warrior on seeing this cosmic form, trembled with fear.
“nabhaḥspṛśaṃ dīptamanekavarṇaṃ vyāttānanaṃ dīptaviśālanetram। dṛṣṭvā hi tvāṃ pravyathitāntarātmā dhṛtiṃ na vindāmi śamaṃ ca viṣṇo॥“ (MBH 6:35:24).
“Indeed, touching the very skies, of blazing radiance, many-hued, mouth wide-open, with eyes that are blazing and large, beholding thee, O Vishnu (all pervading one), with (my) inner soul trembling (in fright), I can no longer command courage and peace of mind”.
In Bhagawad Gita, it's clearly stated that the cosmic form of Bhagawad Gita was not displayed before and No one after Arjuna would ever be able to see that form, even if one does penances, sacrifices, or any other meritorious activities.
“śrībhagavānuvāca mayā prasannena tavārjunedaṃ rūpaṃ paraṃ darśitamātmayogāt। tejomayaṃ viśvamanantamādyaṃ yanme tvadanyena na dṛṣṭapūrvam॥ 47 na vedayajñādhyayanairna dānairna ca kriyābhirna tapobhirugraiḥ। evaṃrūpaḥ śakya ahaṃ nṛloke draṣṭuṃ tvadanyena kurupravīra॥ 48 mā te vyathā mā ca vimūḍhabhāvo dṛṣṭvā rūpaṃ ghoramīdṛṅmamedam। vyapetabhīḥ prītamanāḥ punastvaṃ tadeva me rūpamidaṃ prapaśya॥ 49” (MBH 6:35:47-49).
“The Holy One said,‘Pleased with thee, O Arjuna, I have, by my yogic power, shown thee this supreme form, full of glory, Universal, Infinite, Primeval, which hath been seen before by none save thee. ‘Except by thee alone, hero of Kuru’s race, I cannot be seen in this form in the world of men by anyone else, (aided) even by the study of the Vedas and of sacrifices, by gifts, by actions, (or) by the severest austerities. Let no fear be thine, nor perplexity of mind at seeing this awful form of mine. Freed from fear with a joyful heart, thou again see Me assuming that other peaceful form”.
I hope this clarifies all your queries. Prd..