The seventh day of Navratri is dedicated to Kaalratri devi. Let us find out who Kaalratri devi is, and what her story tells is...
Shumbha and Nishumbha, the two asura brothers had done intense penance and pleased Brahma to get a boon that no man could destroy them.
They then went ahead and defeated Indra and the gods to conquer all the realms. The gods then prayed to Shiva for help, who in turn advised them to seek Parvati's aid. Parvati then took the form of Durga to fight the brothers. In the battlefield, the asuras first sent Chanda and Munda to fight the goddess.
Durga then took the form of Kali (or Kaalratri) and defeated Chanda and Munda easily! She therefore was also bestowed the name Chamunda.
Then, another asura called Raktabija was sent out to kill Kali. He had the boon to produce a double of himself instantly with every drop of his blood that was spilt.
To defeat him, Kali then spread her tongue all over the battlefield in such a way that not a drop of his blood fell to the ground.
Durga attacked him and Kali ensured that no blood fell to the ground. And this way, Raktabij was slayed.
Eventually, Shumbha and Nishumbha were also killed by the goddess.
The seventh day of Navratri is dedicated to Kaalratri devi.