Considered the most ferocious form of Goddess Durga, Kalaratri is revered on Saptami. It is believed that Parvati removed her fair skin to kill the demons Sumbha and Nisumbha. On Saptami, the Goddess appears with rage in her fiery eyes, her skin turning black.
Ultimately, Prāṇa, like everything else, is consciousness which, as Śakti, limits itself in forms which it first creates and sustains; then builds up into other more elaborate forms and again sustains until their life-period is run. All creation and maintenance is a limiting power, with the appearance of unconsciousness, in so far as, and to the degree that, it confines the boundless being-consciousness-bliss; yet that Power is nothing but Consciousness negating and limiting itself. The Great Mother (Śrī Mātā) limits Her infinite being in and as the universe and maintains it. In so far as the form and its life is a limited thing, it is apparently unconscious, for consciousness is thereby limited. At each moment there is creation, but we call the first appearance creation, and its continuance, through the agency of Prāṇa, maintenance. But both that which is apparently limited and that whose operation has that effect is being-consciousness.