The two-armed “Black-Cloaked Mahakala” 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 Mahākāla is technically the primary protector. Six-Armed Mahākāla is also a common Dharmapala in the Kagyu school.