Mahakala — 'great time', the Great Black One — is the chief of the Tibetan dharmapalas, the fierce protectors of the tantras, and in the traditions that hold him he is an emanation of Avalokiteshvara rather than a separate deity; the six-armed form (Shadbhuja) and the two-armed Bernakchen of the Karmapas are the two best known. In the Sakyapa he is Panjaranatha, the protector associated with the Hevajra tantra, and in the Guhyasamaja cycle he guards the mandala. He is depicted in the charnel grounds, black and garlanded with skulls, holding the skull cup and flaying knife, because the iconography displays what the practice cuts through. The name is also a Shaiva epithet, and in Japan the same character became Daikokuten, god of wealth — two developments distinguished from the Tibetan protector here.
Modern Practice
Practitioners approach him only with an empowerment and a teacher — protector practice is transmitted, not adopted from a book — usually with black tea and torma at the end of a session, and he is relied on to clear obstacles to study and practice. The Western 'dark god' reading is a misreading: the blackness is the colour of the absolute, not of malice.