The most horrendous appearance of Durga is in form of Kalaratri. Devi Kalratri is the seventh aspect of Durga and Worshiped among Navdurga. She is believed to be the most violent Devi among Navdurga. She has dark krishna (black) complexion and her appearance is extremely fearful. Her large red eyes, open red tongue, and sobre on hand violently demolish devils and monsters. She is also glorified with name ‘Kali Maa’. Goddess Kaalratri is widely regarded as one of the many destructive forms of Mother Goddess – Kali, Mahakali, Bhadrakali, Bhairavi, Mrityu, Rudrani, Chamunda, Chandi and Durga. Devi Kalaratri has three round eyes, and scattered long black hair. She wears a bead that gleams like fire of electric power. Durga manifested herself as completely destroy the dirt and demons from existence.
She is revered with name Kalratri, because she represents herself as a death of Kaal appeared like a dark night for demons. Donkey is her procession, as she rides on gadarbh. Goddess Kalaratri also has four arms. She holds sobre in her upper left hand and thorn of iron in lower left hand. Her upper right hand is in Var Mudra and lower right hand is in Abhay Mudra. Her splendor is limitless. She removes any kind of fear in life. During seventh day of Navaratri, Sadhak or devotee enters in Sahastrar chakra. After immersing oneself in Sahastrar Chakra, devotees are able to see the entrance of getting complete perfection in Universe.
However, appearance of Maa Kalratri is so dreadful, but her grace is generous toward devotees. Worshipping Maa Kalaratri on the seventh day gives abundant fruitful result. With her grace, devotees get power, position and eminent status on earth. She protects her devotees against evil influence of demons and wrong people. Worshiping her devotees get complete protection and long life.
According to Hindu mythology there is a story behind origin of Kalratri which is depicted below:
In Hindu mythology, it is believed that it is only Goddess Kalratri who could demolish the extreme agony of devil Raktabeej. No one could kill Raktabeej, as even one drop of his scattered blood was able to form another embodiment of Raktabeej. And that way, scattered blood of Raktabeej formed other billions of Raktabeej. Devi Bhagwati manifested herself in Kalaratri, and kept a Patra (vessel) to keep and imbibe entire blood of his body. Her extreme violent appearance while destroying Raktabeej and other demons hugely terrified all organisms of Universe. But still, she is a generous Goddess for her great devotees.
Legend behind Goddess BhardraKali:
Bhadrakali is considered to be a form of Goddess Parvathi who is the respectful wife of Lord Shiva in his Rudra form and it is also said in Puranas and mythology that Bhadrakali is the form of Parvathi taken to calm down Shiva when he takes the form of ViraBhadra. According to the Vayu Purana and the stories from Mahabharata, When Daksha, the demon insulted Lord Shiva during the Great Ashwamedha Yagna, Sati unable to bear the trauma sacrificed her life by falling into the pyre. When Shiva learnt about this, he took the form of ViraBhadra and to add to his power, Goddess Parvathi took the form of Maha Maya Bhadrakali.
There is another story that goes back to Darika who secured a boon from Lord Brahma that he would be excused from any type of death and he would remain unharmed by any form of death and destruction from a man. When the boon was granted, he started harassing and taking over the world and when Lord Shiva learnt about this, he created the Goddess Bhadrakali to kill the demon and gain the victory and peace over the world. He opened his fiery third eye from which the Goddess Bhadrakali appeared to destroy Darika and she sought the help of bloodthirsty ghosts and spirits who hail there. She successfully assassinated Darika as his boon did not say that he cannot be killed by a woman. She returns Kailash by holding the head of Darika.