Once upon a time, two demons Sumbho and Nishumbho plagued the Earth and the Heaven. They posed a challenge to Indra, the king of Gods and his kingdom. To stop this the gods prayed to Goddess Durga who is the eternal Shakti of the Universe. In answer to their prayers, Goddess Kali was born from the forehead of Goddess Durga.
Kali killed the errant asuras with a ‘khara’ which is kind of twisted sword in her hand. Once she started slaying the demons, she could no longer stop. Any demons who crossed her path were slayed. She then made a garland of the heads of dead demons and wore around her neck. To pacify her anger, her husband Shiva lay down in her path. When she stepped on her husband in her fury, she stuck her tongue out in regret. Her fury ended and particular image is portrayed with her tongue stuck out and her foot on Shiva.