Kali is the fearful and ferocious form of the mother goddess. She assumed the form of a powerful goddess and became popular with the composition of the Devi Mahatmya, a text of the 5th - 6th century AD. Here she is depicted as having born from the brow of Goddess Durga during one of her battles with the evil forces. As the legend goes, in the battle, Kali was so much involved in the killing spree that she got carried away and began destroying everything in sight.
To stop her, Lord Shiva threw himself under her feet. Shocked at this sight, Kali stuck out her tongue in astonishment, and put an end to her homicidal rampage. Hence the common image of Kali shows her in her mêlée mood, standing with one foot on Shiva's chest.