Mahagauri is the eighth manifestation of goddess Durga and amongst the Navadurgas. Mahagauri is worshipped on the eighth day of Navratri. According to Hindu mythology, Goddess Mahagauri has the power to fulfill all the desires of her devotees. The one who worships the goddess gets relief from all the sufferings in life. Mahagauri has four arms. Her right arm is in the pose of allaying fear and right lower hand holds a trident in it. She holds a tambourine in her left upper arm and the lower one is in the form of a blessing.
Etymology The name Mahagauri means extremely white, as she was white in colour and very beautiful(Maha, महा = great; Gauri, गौरी = white). Mahagauri is usually depicted with four hands, the hands holding a trident, lotus and drum, while the fourth is in a blessing gesture. The lotus is sometimes replaced with a rosary. She rides a white bull, usually shown wearing white clothes.
The task for the assassination of demons after she has gotten her white complexion back runs as follows: For the killing of demons Shumbha and Nishumbha, it was important for Parvati to have an unborn female child, who would kill them. Hence, as advised by Brahma, Mahadev made her skin complexion black by his magic and also teased her as Kali. Getting agitated by this, Parvati did severe penance of Brahma to obtain her fair complexion. She succeeded in her penance and was advised by Brahma to take bath in Mansarovar River in Himalaya. As she took a bath, her dark skin separated from her and took the form of a female. She was Kaushiki, being born from the cell of Parvati/Gauri.
As a result of which Parvati got her white fair complexion back. Still, for the task of demons' assassination, she then and there, gave her fair complexion to Kaushiki and she(Parvati) again attained the form of Kali. Goddesses Saraswati and Lakshmi provided their powers to Kali as a result of which Kali transformed into Chandika(Chandraghanta). Chandika killed demon Dhumralochan. Chanda and Munda were killed by Goddess Chamunda who appeared out from the third eye of Chandika. Chandika had transformed into Kaalratri again to kill Raktabija and Kaushiki had killed Shumbh and Nishumbh, after which she got merged with Kali to transform her back into Gauri.
Hence, Goddess Kaushiki too got merged with Parvati(Parvati), hence giving her the title of Mahasaraswati Ambika. It is believed that Kali/Durga incarnated as Parvati who was beautiful yet had a black complexion and once, she was teased by Mahadev as Kali. Infuriated by this, she began a great penance to please Lord Brahma and get the fair complexion and in this way, her name Maha Gauri got justified. According to another tale, it is believed that Parvati had got dark by Shiva's magic.
Another story tells that Parvati a dark complexion, but in a different version runs as follows: Goddess Parvati made up her mind to perform an austere penance as advised by Sage Narad so that she might beget Lord Shiva as her husband. So, she renounced all the comforts of the palace and started performing penance in the forest. Her severe penance continued for many years. She braved heat and cold, rains and drought and terrible storms. Her body was covered with dust, earth, soil and leaves of trees. She had developed a blackish skin over her body. At last, Lord Shiva appeared before her and gave his solemn word that he would marry her. He bathed her vigorously by the holy waters of the Ganga emanating from his matted hair. The holy and sacred waters of Ganga washed off all the dirt clinging to Parvati’s person and she became white-complexioned and glorious. Thus by virtue of having acquired a white complexion, Parvati came to be known as Mahagauri(Extremely Fair).