On the day of Deepawali, along with the pan Indian Laxmi Puja, another Goddess is worshipped for the purpose of destroying all evil that is in the outside world and the world within us. She is altogether ferocious, associated with eternal energy, and is literally the redeemer of the universe. She is the Goddess Kali. Also known as Kalika, she is the first among the Dasa Mahavidyas, the ten fierce Tantric Goddesses. Her name is derived from Kala, which is name for Lord Shiva, meaning black, time, and death. It's because of this that she's also referred to as the consort of Lord Shiva.
The popular image that depicts Goddess Kali, wearing a garland of decapitated heads of demons and standing with one foot on the chest of Lord Shiva, originates from a legend. It's a legend, according to which, in the mythological times, two demons named Shambhu and Nishambhu, started creating havoc and disturbed the peace of Indra, the King of Gods, and his empire in heaven. Over a period, the demons grew stronger and the Gods had to take refuge in the holy mountains of Himalayas, the home of Lord Shiva and Parvati. There, they sought help from Goddess Durga.
To restore peace in heaven and on earth, Kali was born, out of Durga's forehead. Kali, with her two escorts: Dakini and Jagini; set out to end the war by killing the demons. After slaughtering all the demons, she made a garland of slain demon heads and wore it around her neck. But in her mission, she lost control and started killing anyone who came in her way. To put an end to this carnage, Lord Shiva devised a plan to come in between the rampaging path of Kali. It's when Kali unknowingly stepped on him that she recovered her senses.
This is the moment depicted in many images with her tongue hanging out, when she accidently stepped on Lord Shiva and repented. Although the legend is ancient, the festival is not from the ancient times and stared being celebrated only at the dawn of 18th century in Bengal by King Krishnachandra of Navadvipa. In the 19th century, the festival began to be celebrated on a grand scale. It was King Krishnachandra's grandson, Ishwarchandra, along with elitist and wealthy landowners of the Bengal who took the tradition forward. And slowly, Kali Puja, along with Durga Puja became the biggest Goddess worship festival in Bengal.
It has now become one of the most powerful forms of goddess worship in India, more so in the parts of Bengal, Orissa, and Assam. In spite of her fearful appearance, the worshippers share a very loving and intimate bond with her, like that of a mother and her child. Believed to be a manifestation of the supreme power, Goddess Kali is worshipped by many at various temples, shrines, and pilgrimages dedicated to her. But more than on any other day, it's the Kali Puja, a worship ritual celebrated on the new moon day of the Hindu month of Ashwin in Bengal, on the night of Kartik Amavasya in October or November, which is considered the greatest form of Kali worship.