The exposition posted below on Lord Yama’s identity is taken from the open source article: JOHN BOWKER. “Yama.” The Concise Oxford Dictionary of World Religions. 1997. Encyclopedia.com. (February 18, 2013):
Yama (Skt., ‘restraint’).
1. The god of death in Hinduism and Buddhism, also called Dharma Rāja, possibly connected with the Iranian Yima. In the Rig Veda he appears in books 1 and 10 presiding over the ancestors or ‘fathers’ (pitra) in the third (highest) heaven of the sky (svarga) realm (above atmosphere, bhuvah, and earth, bhūr). In the Kaṭha Upaniṣad, Yama bestows highest knowledge. Post-Vedic mythology in contrast portrays Yama as a judge and punisher of the dead in a lower world where the soul (ātman, jīva, purusha) goes after death and receives its sentence. The Mahābhārata depicts Yama as clothed in red with glaring eyes, holding a noose with which to bind the souls of the dead. This image is embellished in later mythology where he is a terrible deity inflicting torture upon souls. Yama is associated with the south, the realm of the dead.
In Buddhism, Yama is the Lord of the Underworld. In some respects, he is replaced by Māra. The canonical account of Yama is contained mainly in the two almost identical Devadūta Suttas in Majjhima Nikāya 3. 179 ff., and Anguttara Nikāya 1. 138 ff.
In the post-canonical Buddhist literature, Yama is depicted as the overlord of the purgatory system who assigns to beings the punishments they must undergo in expiation of their karmic misdeeds. In Tantric Buddhism, Yama is a fierce deity. Tibetan iconography and the Tibetan Book of the Dead (Bardo Thodol) portray Yama, who appears at death, as standing in a halo of flames, adorned with human skulls and heads, holding in his left hand the mirror of karma (which reflects the good and bad deeds of the deceased) and in his right hand the sword of wisdom (prajña).
2. The first limb of eight-limbed (aṣṭaṅga) or rāja yoga comprising five ethical rules: (i) non-injury (ahiṃsā), (ii) truthfulness (satya), (iii) non-stealing (asteya), (iv) celibacy (brahmacarya), and (v) greedlessness (aparigraha). Commitment to these is the Great Vow (Mahāvrata).