In Kali's most famous myth, Durga and her assistants, Matrikas, wound the demon Raktabija, in various ways and with a variety of weapons, in an attempt to destroy him. They soon find that they have worsened the situation, as for every drop of blood that is spilt from Raktabija, the demon reproduces a clone of himself. The battlefield becomes increasingly filled with his duplicates. Durga, in need of help, summons Kali to combat the demons. It is also said that Goddess Durga takes the form of Goddess Kali at this time.
The Devi Mahatmyam describes:
Out of the surface of her (Durga's) forehead, fierce with frown, issued suddenly Kali of terrible countenance, armed with a sword and noose. Bearing the strange khatvanga (skull-topped staff ), decorated with a garland of skulls, clad in a tiger's skin, very appalling owing to her emaciated flesh, with gaping mouth, fearful with her tongue lolling out, having deep reddish eyes, filling the regions of the sky with her roars, falling upon impetuously and slaughtering the great asuras in that army, she devoured those hordes of the foes of the devas.
Kali destroys Raktabija by sucking the blood from his body and putting the many Raktabija duplicates in her gaping mouth. Pleased with her victory, Kali then dances on the field of battle, stepping on the corpses of the slain. Her consort Shiva lies among the dead beneath her feet, a representation of Kali commonly seen in her iconography as Daksinakali.
In the Devi Mahatmya version of this story, Kali is also described as a Matrika and as a Shakti or power of Devi. She is given the epithet Cāṃuṇḍā (Chamunda), i.e. the slayer of the demons Chanda and Munda. Chamunda is very often identified with Kali and is very much like her in appearance and habit.
Ref: en.wikipedia.org
.jpg)