Mahākāla

Sanskrit name meaning ‘The Great Black One’. Originally a non-Buddhist deity, sometimes seen as a form of the Hindu god Śiva (see Hinduism), he is a wrathful tutelary deity (yi-dam) and protector of the faith (dharmapāla). In tantric Buddhism he is considered to be a manifestation of the Bodhisattva Avalokiteśvara, and his worship (pūjā) and related practices are described in detail in the Mahākāla Tantra. Various iconographic forms exist of Mahākāla in Indian and Tibetan tantric Buddhism with between four and sixteen arms. A non-wrathful form exists in Japan where he is associated with good fortune and is known as Daikokuten.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)