Mahakala (Sanskrit: महाकाल; IAST: Mahākāla) is a deity common to Hinduism, Buddhism and Sikhism. According to Hinduism, Mahakala is a manifestation of Shiva and is the consort of Hindu Goddess Mahakali and most prominently appears in Kalikula sect of Shaktism. Mahākāla also appears as a protector deity known as a dharmapala in Vajrayana Buddhism, particularly most Tibetan traditions (Citipati), in Tangmi (Chinese Esoteric Buddhism) and in Shingon (Japanese Esoteric Buddhism). He is known as Dàhēitiān and Daaih'hāktīn (大黑天) in Mandarin and Cantonese, Daeheukcheon (대흑천) in Korean, Đại Hắc Thiên in Vietnamese, and Daikokuten (大黒天) in Japanese. In Sikhism, Mahākāla is referred to as Kal, who is the governor of Maya.
According to Shaktisamgama Tantra, the spouse of Mahakali is extremely frightening. Mahakala has four arms, three eyes and is of the brilliance of 10 million black fires of dissolution, dwells in the midst of eight cremation grounds. He is adorned with eight skulls, seated on five corpses, holds a trident, a drum, a sword and a scythe in his hands. He is adorned with ashes from the cremation ground and surrounded by numbers of loudly shrieking vultures and jackals. At his side is his consort symbolized as Kālī (काली, “the power of time”). Both Mahakala and Kālī (काली, “the power of time”) represent the ultimate destructive power of Brahman and they are not bounded by any rules or regulations. They have the power to dissolve even time and space into themselves and exist as Void at the dissolution of the universe. They are responsible for the dissolution of the universe at the end of Kalpa. They are also responsible for annihilating great evils and great daemons when other gods, Devas and even Trimurtis fail to do so. Mahakala and Kali annihilates men, women, children, animals, the world and the entire universe without mercy because they are Kala or Time in the personified form and Time is not bound by anything and Time does not show mercy, nor does it wait for anything or anyone.  In some parts of Odisha, Jharkhand and Dooars, (that is, in northern Bengal), wild elephants are worshiped as Mahakala. 
Mahakala is typically black in color. Just as all colors are absorbed and dissolved into black, all names and forms are said to melt into those of Mahakala, symbolizing his all-embracing, comprehensive nature. Black can also represent the total absence of color, and again in this case it signifies the nature of Mahakala as ultimate or absolute reality. This principle is known in Sanskrit as "nirguna", beyond all quality and form, and it is typified by both interpretations.
Mahayana Buddhism, and all schools of Tibetan Buddhism, rely on Mahakala as guardian deity. He is depicted in a number of variations, each with distinctly different qualities and aspects. He is also regarded as the emanation of different beings in different cases, namely Avalokiteśvara (Wylie: spyan ras gzigs) or Cakrasaṃvara (Wylie: ’khor lo bde mchog). Mahakala is almost always depicted with a crown of five skulls, which represent the transmutation of the five kleśās (negative afflictions) into the five wisdoms.
The most notable variation in Mahakala's manifestations and depictions is in the number of arms, but other details can vary as well. For instance, in some cases there are Mahakalas in white, with multiple heads, without genitals, standing on varying numbers of various things, holding various implements, with alternative adornments, and so on.
Nyingshuk came from Khyungpo Nenjor, the founder of the Shangpa Kagyu, and spread to all the lineages—Sakya, Nyingma, and Gelug—as well as various Kagyu lineages. There are also terma lineages of various forms of Six-Armed Mahakala. Nyinghsuk, though derived from the Shangpa, is not the major Shangpa one—it is in a dancing posture rather than upright, and is a very advanced Mahakala practice.
Various Four-Armed Mahakalas (Skt. Chaturbhūjamahākāla, Wylie: mgon po phyag bzhi pa) are the primary protectors of the Karma Kagyu, Drikung Kagyu, Drukpa Lineage and the Nyingma of Tibetan Buddhism. A four-armed Mahakala is also found in the Nyingma school, although the primary protector of the Dzogchen (Skt: Mahasandhi) teachings is Ekajati.
The two-armed "Black-Cloaked Mahakala" (Wylie: mgon po ber nag chen) is a protector of the Karma Kagyu school clad in the cloak of a māntrika "warlock". His imagery derives from terma of the Nyingma school and was adopted by the Karma Kagyu during the time of Karma Pakshi, 2nd Karmapa Lama. He is often depicted with his consort, Rangjung Gyalmo. He is often thought to be the primary protector, but he is in fact the main protector of the Karmapas specifically. Four-Armed Mahakala is technically the primary protector. Six-Armed Mahakala (Wylie: mgon po phyag drug pa is also a common dharmapala in the Kagyu school.
Pañjaranātha Mahakala "Mahakala, Lord of the Tent", an emanation of Mañjuśrī, is a protector of the Sakya school.
Mahakala is also known as Mahakala Bhairava and Kala Bhairava in Hinduism, and many temples in India and Nepal are dedicated solely for Mahakala Bhairava, for example at the temple in Ujjain, which is mentioned more than once by Kālidāsa. The primary temple, place of worship for Mahakala is Ujjain. Mahakala is also a name of one of Shiva's principal attendants (Sanskrit: gaṇa), along with Nandi, Shiva's mount and so is often represented outside the main doorway of early Hindu temples.
In Japan, this deity is variously considered to be the god of wealth or of the household, particularly the kitchen. He is recognised by his wide face, smile, and a flat black hat, in stark contrast to the fierce imagery portrayed in Tibetan Buddhist art. He is often portrayed holding a golden mallet, otherwise known as a magic money mallet, and is seen seated on bales of rice, with mice nearby (mice signify plentiful food).