Mahakala, Chaturmukha (Four-faced Great Black One) associated with the Guhyasamaja Tantra along with the Twenty-five and Fifty Chapter Mahakala Tantras. There are also Nyingma Revealed Treasure Traditions (terma) of Chaturmukha.
According to the Sakya Tradition this form of Mahakala with four faces and four arms is rarely depicted in art and virtually never shown to those who are uninitiated. The early Sakya Teachers devised a simplified form of the deity to be depicted on paintings as a place card-holder for the true image and form which was considered too secret and dangerous to display publicly. The simplified form is that of what has come to be known as Brahmanarupa Mahakala.