Because Thou devourest Kāla, Thou art Kāli, the original form of all things, and because Thou art the Origin of and devourest all things Thou art called the Adya [the Primordial One].
Re-assuming after Dissolution Thine own form, dark and formless, Thou alone remainest as One ineffable and inconceivable. Though having a form, yet art Thou formless; though Thyself without beginning, multiform by the power of Maya, Thou art the Beginning of all, Creatrix, Protectress, and Destructress that Thou art.
Mahakali, literally translated as Great Kali, is sometimes considered as a greater form of Kali, identified with the Ultimate reality of Brahman, the Absolute Reality Itself. It can also be used as an honorific of the Goddess Kali, signifying her greatness by the prefix "Mahā-". Mahakali, in Sanskrit, is etymologically the feminized variant of Mahakala or Great Time (which is interpreted also as Death), an epithet of the God Shiva in Hinduism.
Mahakali is the presiding Goddess of the first episode of the Devi Mahatmya. Here she is depicted as Devi in her universal form as Shakti. Here Devi serves as the agent who allows the cosmic order to be restored.
Kali is depicted in the Mahakali form as having ten heads, ten arms, and ten legs. Each of her ten hands is carrying a various implement which vary in different accounts, but each of these represent the power of one of the Devas or Hindu Gods and are often the identifying weapon or ritual item of a given Deva.
The implication is that Mahakali subsumes and is responsible for the powers that these deities possess and this is in line with the interpretation that Mahakali is identical with Brahman. While not displaying ten heads, an "ekamukhi" or one headed image may be displayed with ten arms, signifying the same concept: the powers of the various Gods come only through Her grace.