Mahakala is a category of Tantric Buddhist deity. His common function is as a protector (Dharmapala) and specifically the primary Wisdom Protector of Himalayan and Tibetan Buddhism. He is however also a meditational deity (ishtadevata) in the highest Anuttarayoga Tantras. For the study of Mahakala there are three important topics: Principal Texts, Principal Systems and Principal Iconographic Forms
There are dozens of different variations and forms of Mahakala. He is typically in wrathful appearance following the Indian model of a Raksha demon. In most occurrences and uses of Mahakala, he is paired with a meditational deity such as Shri Hevajra and as Panjarnata Mahakala, Chakrasamvara and Chaturbhuja Mahakala, Guhyasamaja and Chaturmukha Mahakala. In most cases Mahakala is an emanation, or wrathful aspect, of the principal meditational deity that he is associated with. For instance Panjarnata Mahakala is the most wrathful emanation, or form, of Shri Hevajra. In other situations Mahakala might be a wrathful emanation of Vajradhara or Akshobhya Buddha. Aside from the individual Anuttarayoga Tantras that teach various forms of Mahakala, the principal texts are the Eight Chapter, Twenty-five Chapter and Fifty Chapter Mahakala Tantras.