Mahabharata 13.146.2

13.146.2

vadantyagniṃ mahādevaṃ tathā sthāṇuṃ maheśvaram |
ekākṣaṃ tryambakaṃ caiva viśvarūpaṃ śivaṃ tathā || 2 ||

"Vasudeva said, 'O mighty-armed Yudhishthira, listen to me as I recite to thee the many names of Rudra as also the high blessedness of that high-souled one. The Rishis describe Mahadeva as Agni, and Sthanu, and Maheswara; as one-eyed, and three-eyed, of universal form, and Siva or highly auspicious. Brahmanas conversant with the Vedas say that that god has two forms. One of these is terrible, and the other mild and auspicious. Those two forms, again, are subdivided into many forms. That form which is fierce and terrible is regarded as identical with Agni and Lightning and Surya. The other form which is mild and auspicious is identical with Righteousness and water and Chandramas. Then, again, it is said that half his body is fire and half is Soma (or the moon). That form of his which is mild and auspicious is said to be engaged in the practice of the Brahmacharya vow. The other form of his which is supremely terrible is engaged in all operations of destruction in the universe. Because he is great (Mahat) and the Supreme Lord of all (Iswara), therefore he is called Maheswara. And since he burns and oppresses, is keen and fierce, and endued with great energy, and is engaged in eating flesh and blood and marrow, he is said to be Rudra. Since he is the foremost of all the deities, and since his dominion and acquisitions are very extensive, and since he protects the extensive universe, therefore he is called Mahadeva.