here are at least six different traditions of the Six-armed Mahakala (shadbhuja). The most famous and well-known, produced more as art, is the Shangpa Kagyu tradition Mahakala. This form of the deity was introduced to Tibet by Khedrub Khyungpo Naljor in the 12th century. It was later adopted by the Bulug and Jonang schools as an alternate form of Mahakala. In the 15th century the Shangpa form was also adopted by the new Gelug tradition of Je Tsongkapa and became the chief protector of that school.
The two forms arising from the Fifty Chapter Mahakala Tantra and the Northern Treasure Tradition both have three faces and six hands.
The Shangpa, Dorje Shanglon and Chogyur Lingpa forms of the deity each have one face and six hands. In addition, the Shangpa form has numerous branch forms of the deity such as the more common white, green, yellow and red forms.