Fiercest and the most fearful form of Maa Parvati is that of Kaalratri, when She removed her outer golden skin to kill demons Shumbha and Nishumbha.
According to the legends, two demons, Shumbha and Nishumbha, invaded devalok and defeated the demigods. Lord Indra, along with the demigods went to Lord Shiva for the help. They prayed to Maa Parvati (also an incarnation of Maa Durga). She heard the prayer while she was bathing and hence created another goddess, Chandi to help the demigods in conquering the demons. Shumbha and Nishumbha sent demon generals, Chanda and Munda, Goddess Chandi created a dark goddess Kaalratri to kill them. Hence, She is also known as Chamunda.
Following that, a demon named Raktabija, who had the boon that if a drop of his blood falls on the ground, a clone of his would appear. When Maa Kaalratri attacked him, his spilt blood created several clones, making it difficult to defeat him. Furious at this, Maa Kaalratri drank his blood to avoid it falling on the ground, thereby killing him and thus helping Maa Chandi to kill Shumbha and Nishumbha.
Maa Kaalratri rides a donkey and is depicted with four hands, where Her right hands are in Abhayamudra (blessing hand posture) and other in Varadamudra (posture dispensing boons) and carries a sword and a deadly iron hook in left hands.
Each day of the Navratri is devoted to one of the nine avatars of Maa Durga. The nine different forms are Shailaputri, Brahmacharini, Chandraghanta, Kushmanda, Skandamata, Katyayini, Kaalratri, Mahagauri and Siddhidhatri.