Ma Kalarathri worshipped on the 7th day is dark as a night and so the name. Her hair cascades around her with a necklace adorning her neck flashing like a lightning. She is the destroyer of darkness and ignorance. Sitting astride a donkey, Ma Kalarathri blesses her devotees with prosperity and so is also felicitated as Sbhubamkari. Mata is a chaturbhuja avatar holding weapons in her left hands and showing abhaya varadha mudras with her right hands. Yogis and sadhakas meditate on Shahtra chakra on the 7th day. Kalarathri is the most ferocious of the avatars. In a part of the Devi Mahatmyam is narrated the story of the fight between the demon Raktbij and the Divine mother. From the divine mother emanates the form Chamunda or Kali or Kalratri. When the demon Raktbij was attacked, and he bled, his blood would fall to the earth, and another army of fresh Raktbij demons would be born. So the divine mother asks Chamunda to drink the blood of the demon so that he cannot multiply. And Chamunda attacks him and drinks his blood till the demon is fully destroyed. Kalarathri is one of the Devi standing guard over Varanasi.
Ya devi sarva bhuteshu Maa Kalratri rupen Sanisthita,
Namstasye, namastaye, namstaye namoh namah.