Dattatreya
Indeed, the Dattatreya Upanisad, which opens proclaiming Dattatreya's identity with Vishnu, ends with the mantra Om Namah Shivaya, identifying Datta with Shiva. In the last portion of the third chapter, Mahesvara (Shiva) alone is said to pervade reality and shine in every heart of man. He alone is in front, behind, to the left, to the right, below, above, everywhere the center. Finally, Mahesvara is identified with Dattatreya, depicting the latter as an Avatar of Shiva.