simultaneously brilliant and terrifying form

Having gained a "divine vision" it is difficult to narrate the "vision" in words!

Sanjaya (the narrator of Srimad Bhagavad Gita to Dhritarastra - "sanjaya" means the one who has completely conquered his external and internal nature) describes the vision in most brilliant way as follows:

दिवि सूर्यसहस्रस्य भवेद्युगपदुत्थिता।
यदि भाः सदृशी सा स्याद्भासस्तस्य महात्मनः।।11.12।।
11.12 दिवि in the sky, सूर्यसहस्रस्य of a thousand suns, भवेत् were, युगपत् at once (simultaneously), उत्थिता arisen, यदि if, भाः splendour, सदृशी like, सा that, स्यात् would be, भासः splendour, तस्य of that, महात्मनः of the mighty Being (great soul).

तत्रैकस्थं जगत्कृत्स्नं प्रविभक्तमनेकधा।
अपश्यद्देवदेवस्य शरीरे पाण्डवस्तदा।।11.13।।
11.13 तत्र there, एकस्थम् resting in one, जगत् the universe, कृत्स्नम् the whole, प्रविभक्तम् divided, अनेकधा in many groups, अपश्यत् saw, देवदेवस्य of the God of gods, शरीरे in the body, पाण्डवः son of Pandu, तदा then.

ekastham (resting in one) pravibhaktam anekadhaa (divided in many groups) is a truly graphic description of the viswa roopa by Sanjaya. It has to be realized in the transcendental state of samadhi not by the "mind"!

at the same time Arjuna starts seeing the Ugra Rupam of Lord:
अनादिमध्यान्तमनन्तवीर्य-
मनन्तबाहुं शशिसूर्यनेत्रम्।
पश्यामि त्वां दीप्तहुताशवक्त्रम्
स्वतेजसा विश्वमिदं तपन्तम्।।11.19।।
11.19 अनादिमध्यान्तम् without beginning, middle or end, अनन्तवीर्यम् infinite in power, अनन्तबाहुम् of endless arms, शशिसूर्यनेत्रम् Thy eyes as the sun and the moon, पश्यामि (I) see, त्वाम् Thee, दीप्तहुताशवक्त्रम् Thy mouth as the burning fire, स्वतेजसा with Thy radiance, विश्वम् the universe, इदम् this, तपन्तम् heating.

Arjuna describes his experience with the Ugra Roopa and concludes his experience as follows:

11.30 लेलिह्यसे (Thou) lickest, ग्रसमानः devouring, समन्तात् on every side, लोकान् the worlds, समग्रान् all, वदनैः with mouths, ज्वलद्भिः flaming, तेजोभिः with radiance, आपूर्य filling, जगत् the world, समग्रम् the whole, भासः rays, तव Thy, उग्राः fierce, प्रतपन्ति are burning, विष्णो O Vishnu!

So, Arjuna's perception of the viswaroopa is slightly different from the Sanjaya's perception. Why is it so?

Arjuna is still not completely conquered his Mind. One who has not completely conquered his mind can't get established in the natural unseeded samadhi. Even if the "divine vision" is granted, they get back to the world vision for more saadhana.

om tat sat