Mhalsa Devi is a manifestation of Mother Goddess Shakti. She
appeared as the consort of Khandoba, a manifestation of Lord Shiva. This form
of Goddess Shakti is widely worshipped in Maharashtra, Goa, Karnataka and
Telangana.
Two demons named Manni and Malla took the form of wolves and
harassed shepherds and farmers. The shepherds and farmers prayed to Lord Shiva,
who took the form of Khandoba. He then appeared along with Mhalsa Devi and
vanquished the demons.
Mhalsa Devi is worshipped as a four-armed goddess. Mhalsa is also often depicted with two arms and accompanying Khandoba on his horse or standing besides him.
Temples dedicated to Mhalsa Devi is mostly along with that of
Khandoba. There are also shrines in some regions that is exclusively dedicated
to Mhalsa Devi.
At the most famous temple dedicated to Khandoba in Jejuri, Mhalsa Devi is worshipped as his consort.
She is the Kuldevi or family deity of thousands of families in western parts of India.
Legend has it that Shiva was enchanted by Mohini avatar of Vishnu. She promised him to be his wife in her earthly incarnation (avatar) when he would be incarnated as Khandoba on earth.
According to another legend, Mhalsa is an avatar of Goddess Parvati. As per this legend, Mhalsa was born as the daughter of Timmaseth, a Lingayat merchant in Newasa. Khandoba came in the dream of the merchant and asked him to marry Mhalsa to Shiva. She was married to Khandoba on Paush Purnima (the full moon day in January) in Pali (Pembar). Two shivlings appeared on this occasion. An annual festival marking this event is celebrated in Pali every Paush Purnima.
Please note that there is another Goddess known as Mahalasa Narayani. She is considered as a form of Mohini, the female avatar of Bhagwan Vishnu. This form is worshipped mainly in Goa. Mahalasa Narayani form is not associated with Shiva.