On day 7, Mā Durgā takes the 7th form as Mā Kālarātri, one of the fiercest forms of Durgā and her appearance evokes fear. ‘Kāla’ means ‘death & time’, and ‘Rātri’ means ‘night’. This form is believed to be the destroyer of all demonic qualities and negative energies, who flee upon knowing of her arrival. She killed the demons Śumbha & Niśumbha.

Spiritually, after worshipping three aspects of Mā Durgā on the first three days to remove impurities from the mind, Mā Lakṣmi is worshipped on the next three days, to cultivate the noble values making the mind ready to absorb the supreme knowledge which is done by worshipping Mā Sarawati on last three days.
Kālarātri is the one who destroys ignorance and removes darkness so that the devotee merges into the supreme consciousness. This form primarily depicts that life also has a dark side – the violent Mother Nature who nurtures life can also create havoc and remove all dirt.
Śumbha & Niśumbha once invaded the abodes of the gods. Indra sought Bhagwān Śiva’s help in taking back their kingdoms. They all prayed to Mā Pārvati who was bathing. She meditated on and called her another form Chandi (Ambikā) to assist the gods by conquering the demons.
Meanwhile, Chanḍa and Munḍa, the generals of Śumbha & Niśumbha came to disturb the gods again. Devi Chandi blessed by Mā Pārvati created a dark goddess Kālī to kill those generals. Kālī killed them, gaining the name of Chamunḍā (killer of Chanḍa and Munḍa). Hearing the death of Chanḍa and Munḍa, Raktabīja arrived to take revenge. He had a boon that a drop of blood from his body spilling into the ground would create his close equally strong.
It was now almost impossible to kill Raktabīja. Kālī (Kālarātri) made countless attempts to kill him but from his blood drops many more emerged. In a rage, she started drinking his blood to prevent it from dropping to the ground. Then his commanders Śumbha & Niśumbha arrived. They began beating Chamunḍā.
Kālī soon raged and went berserk in killing whoever came in her way. Both the commanders were killed. Scared to death, the gods prayed to Bhagwān Śiva to calm her down. Bhagwān Śiva appeared before her and lay down to get run over by her feet. Seeing her beloved husband beneath her foot, she felt embarrassed and bit her tongue. She stopped and fell unto the feet of Bhagwān Śiva. The complexion of Mā Kālarātri is the dark night with bountiful hair and heavenly shaped form, and she has four hands. The left two hands hold a cleaver and a torch, and the right two are in the mudras of “giving” and “protecting”. She wears a necklace that shines like the moon. Kālarātri has three eyes that emanate rays like lightning. Flames appear through her nostrils when she inhales and exhales. Her mount is the donkey.