Argthtjtr

In Mahabharata, Krishna/Vishnu shows his viswaroopa 4 times. Is there a similar instance of Shiva showing his viswaroopa? Question is specific to Mahabharata.

Yes.

http://www.sacred-texts.com/hin/m07/m07197.htm

"Vyasa said, 'O highly significant is this matter that thou enquirest of me from surprise. I will tell thee everything; listen attentively. He that is called Narayana is older than the oldest ones. For accomplishing some purpose, that creator of the universe took his birth as the son of Dharma. On the mountain of Himavat he underwent the severest ascetic austerities. Endued with mighty energy, and resembling fire or the sun (in splendour), he stood there with arms upraised. Possessed of eyes like: lotus-petals, he emaciated himself there for sixty-six thousand years, subsisting all the while upon air alone. Once more undergoing severe austerities of another kind for twice that period, he filled the space between earth and heaven with his energy. When by those austerities, O sire, he became: like Brahma 1 he then beheld the Master, Origin, and Guardian of the Universe, the Lord of all the gods, the Supreme Deity, who is exceedingly difficult of being gazed at, who is minuter than the minutest and larger than, the largest, who is called Rudra, 2 who is the lord of all the superior ones, who is called Hara and Sambhu, who has matted locks on his head, who is the infuser of life into every form, who is the First cause of all immobile: and mobile things, who is irresistible and of frightful aspect, who is of fierce wrath and great Soul, who is the All-destroyer, and of large heart; who beareth the celestial bow and a couple of quivers, who is cased in golden armour, and whose energy is infinite, who holdeth Pinaka, who is; armed with thunderbolt, a blazing trident, battle axe, mace, and a large sword; whose eye-brows are fair, whose locks are matted, who wieldeth the heavy short club, who hath the moon on his forehead, who is clad in tiger-skin, and who is armed with the bludgeon; who is decked with beautiful angadas, who hath snakes for his sacred thread, and who is surrounded by diverse creatures of the universe and by numerous ghosts and spirits, who is the One, who is the abode of ascetic austerities, and who is highly adored by persons of venerable age; who is Water, Heaven, Sky, Earth, Sun, Moon, Wind and Fire, and who is the measure of the duration of the universe. Persons of wicked behaviour can never obtain a sight of that unborn one, that slayer of all haters of Brahmanas, that giver of emancipation. 3 Only Brahmanas of righteous conduct, when cleansed of their sins and freed from the control of grief, behold him with their mind's eye. In consequence of his ascetic austerities, Narayana obtained a sight of that unfading one, that embodiment of righteousness, that adorable one, that Being having the universe for his form. Beholding that
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supreme Abode of all kinds of splendour, that God with a garland of Akshas round his neck, Vasudeva, with gratified soul, became filled with delight which he sought to express by words, heart, understanding, and body. Then Narayana worshipped that Divine Lord, that First cause of the universe, that giver of boons, that puissant one sporting with the fair-limbed Parvati, that high-souled Being surrounded by large bands of ghosts, spirits, that Unborn one, that Supreme Lord, that Embodiment of the unmanifest, that Essence of all causes, that One of unfading power. Having saluted Rudra, that destroyer of the Asura Andhaka, the lotus eyed Narayana, with emotion filling his heart, began to praise the Three-eyed one (in these words), 'O adorable one, O first of all the gods, the creator of everything (viz., the Prajapatis) who are the regents of the world, and who having entered the earth,--thy first work,--had, O lord, protected it before, have all sprung from thee. Gods, Asuras, Nagas, Rakshasas, Pisachas, human beings, birds, Gandharvas, Yakshas and other creatures: with the entire universe, we know, have all sprung from thee. Everything that is done for propitiating Indra, and Yama, and Varuna, and Kuvera and Pitris and Tvashtri, and Soma, is really offered to thee. Form and light, sound and sky, wind and touch, taste and water, scent and earth, 1 time, Brahma himself, the Vedas, the Brahmanas and all these mobile objects, have sprung from thee. Vapours rising from diverse receptacles of water, becoming rain-drops, which failing upon the earth, are separated from one another. When the time of the Universal dissolution comes those individual drops, separated from one another, once more unite together and make the earth one vast expanse of water. He that is learned, thus observing the origin and the destruction, of all things, understands thy oneness. Two birds (viz., Iswara and Jiva), four Aswatthas with their wordy branches (viz., the Vedas), the seven guardians (viz., the five essences or elements and the heart and the understanding), and the ten others that hold this city (viz., the ten senses that constitute the body), have all been created by thee, but thou art separate from and independent of them. The Past, the Future, and the Present, over each of which none can have any sway, are from thee, as also the seven worlds and this universe. I am thy devoted adorer,--be graceful unto me. Do not injure me, by causing evil thoughts to penetrate my heart. Thou art the Soul of souls, incapable of being known. He that knows thee as the Universal Seed, attaineth to Brahma. Desiring to pay thee respects, I am praising thee, endeavouring to ascertain thy real nature, O thou that art incapable of being understood by the very gods. Adored by me, grant me the boons I desire but which are difficult of acquisition. Do not hide thyself in thy illusion

And in the most amazing bit in all of Hindu scripture he gives a boon to Narayana that if even he fought him (Siva) he (Narayana) would be victorious !

In the same Drona Parva, we have

In the Vedas the excellent hymn called Sata Rudriya, hath been sung in honour of that great God called the infinite Rudra. That God is the lord of all wishes that are human and heavenly. He is omnipotent, and he is the supreme master. Indeed, that God pervadeth the vast universe. The Brahmanas and the Munis describe him as the First-born of all creatures. He is the First of all the gods; from his mouth was born Vayu (the wind). And since he always protecteth the creatures (of the universe) and sporteth with them, and since also he is the lord of all creatures, therefore is he called Pasupati. And since his Phallic emblem is always supposed to be in the observance of the vow of Brahmacharya, and since he always gladden the world, therefore he is called Maheswara. The Rishis, the gods, the Gandharvas, and Apsaras, always worship his Phallic emblem which is supposed to stand upright. That worship maketh Maheswara glad. Indeed, Sankara (at such worship) becomes happy, pleased, and highly glad. And since with respect to the past, the future, and the present, that God has many forms, he is, on that account, called Vahurupa (many-formed). Possessed of one eye he blazeth forth in effulgence, or he may be regarded to have many eyes on every side of his body. And since, he possesseth the worlds, he is for that reason called Sarva. And since his form is like that of smoke, he is for that reason called Dhurjjati. And since those deities, viz., the Viswedevas are in him, he is for that reason called Viswarupa.