Kaalratri is worshipped on the seventh day of Navratri. Kaal Ratri is the one who destroys ignorance and removes darkness of birth, so that the devotee merges into the supreme consciousness with destruction of the darkness of ignorance.
Shumbha and Nishumbha once invaded the abodes of the gods in order to attack and defeat them. Indra sought Lord Shiva's help in taking back their kingdoms. They all prayed to Goddess Parvati who was bathing. She meditated on and called her another form Chandi (Ambika) to assist the gods by vanquishing the demons.
Chanda and Munda the generals of Shumbha and Nishumbha came to disturb the gods again. Goddess Chandi blessed by Parvati created a dark goddess Kali to kill those generals. Kali quickly killed them, gaining the name Chamunda

Hearing the death of Chanda and Munda, Raktabeej arrived to take revenge. He had a boon that a drop of blood from his body spilling into the ground would create his clone equally strong. Devi made countless attempts to kill him but from his blood drops many more emerged. In a rage, she started drinking his blood to prevent it from dropping on the ground.
Kali soon raged and went berserk in killing whoever came in her way. Both the commanders were killed.