Chamunda Kaali is a roopa of Umadevi’s Durga roopa. When the devatas were frustrated by the nuisance of Shumbha and Nishumbha, they approached Kailasa and prayed there to Vishnumaya Durga, who had previously killed Mahishasura. They praised Her as present in every soul in the form of power, intellect, hunger, mother, peace, forgiveness, shyness, sleep/slumber, knowledge, growth, satisfaction, courage, etc. She is present in every soul as these qualities in three forms - Sattvika, Raajasika and Taamasika. That’s why the devatas say, “Namastasyai Namastasyai Namastasyai Namo Namaha”, thus bowing to the three forms of the Devi.
When this was going on, Umadevi was going for a bath when She saw the devatas praying. She thought, “When I am here, whom are these devatas praying to?” Suddenly, a Devi came from Her body cell. She was dark in color and was holding a bell, trident, plough, conch shell, and a cylindrical club, a discus, a bow, and an arrow. This was none other than Mahasaraswati. This is Umadevi’s Durga roopa. Earlier, Umadevi had a dark body, due to which Lord Shiva once mockingly called Her Kaali. She felt bad and did penance to please Lord Brahma, and She got a golden body, after which She was known as Gowry. Now, this dark body didn’t go to waste. It was in Her body cells. The devatas have the power to do so. So, now when these devatas were praising Vishnumaya Durga, this Vishnumaya Durga entered that dark body of Umadevi present in Her cell and came out as Mahasaraswati. That body contained Uma sannidhana; therefore, it is Umadevi’s Durga roopa. Since She came from Umadevi’s body cell (Kosha or cell), She was known as Kaushiki.
Now, when this Chandika Durga (Mahasaraswati) was sitting on Her vehicle, which is the lion, then Shumbha and Nishumbha both saw Her. They were captivated by Her beauty and wanted to marry Her, due to which they sent a messenger to Devi. That messenger came and told Chandika Durga that Shumbha and Nishumbha were attracted by Her beauty and wanted to marry Her. Devi, on hearing this, said, “Out of childishness, in my childhood I had vowed that I would marry the one who could defeat me. If they defeat me, then only I can marry them. Go and tell this to both of the demon kings.”
The messenger went and told everything that Devi told. Both Shumbha and Nishumbha were very angry about this and considered that Devi was being adamant. So, they sent a demon by the name Dhoomralochana. Dhoomralochana came with a large troop to attack Devi. He came running towards Devi in pure anger. Devi reduced him to ashes just by Her humkaara (making of the ‘Hum’ sound). When the news of Dhoomralochana’s death reached Shumbha and Nishumbha, they were very much angry, and they sent Chanda and Munda, named demons, to fight Devi. Here, the lion of Devi started destroying the soldiers. With its nails, it killed the demon soldiers; with its bare hands, it hit the soldiers and beheaded them. The lion pounced on some soldiers and killed them. Thus, the king of lions destroyed many soldiers in this manner.
Now, Chanda and Munda came there to fight. On seeing Devi sitting on the Mount Kailasa, on Her vehicle, the lion, they both ran towards Her to attack Her. At that time, Devi was very angry at them. And from Her forehead came Kaali. She was a very fierce form of Devi, with blood-red eyes and a garland of human heads; Her tongue had come out as She came out, wearing the tiger skin as Her clothes. As She was coming out, She started killing the demons. She pulled out the demons by their legs, heads, etc., and ate them and killed them. She pulled some demons by their hair, some by their neck, some by their legs, some by their thighs, like this, and ate them. This way, Kaali was fiercely fighting with the demons. This Kaali now fought with Chanda and cut his head and killed him. On seeing this, Munda was frightened. He ran from there, but Devi caught him and laughed aloud, beheaded Munda too, and killed him.
Now, this Kaali went to Chandika Durga and told Her that She had killed Chanda and Munda. Chandika Durga said, “Since you killed these two wicked demons, Chanda and Munda, you will be known as Chamunda/Chamundi. That’s why Kaalidevi is known as Chamunda.
यस्माच्चण्डं च मुण्डं च गृहीत्वा त्वमुपागता।
चामुण्डेति ततो लोके ख्याता देवि भविष्यसि॥
Because you have captured and brought to me the severed heads of both Chanda and Munda, you, O Goddess, shall henceforth be known and revered throughout the world by the name Chamunda.
[Markandeya Purana, Devi Mahatmya, Uttara Charita, Chanda Munda Vadha, Shloka 27]
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!