An arrogant and mighty demon Andhakasura tormented the deities in heaven. He was so proud of his immortality, a boon granted to him by Lord Brahma, that no celestial power could destroy him. Lord Shiva offered to help and a fierce battle ensued between the two. When Shiva’s trident attacked him, blood gushed out and every drop took multiple forms of the demon. The Lord was furious and from his mouth fire emanated and resulted in the manifestation of Goddess Yogishwari. The other forms were manifested from other deities totalling eight in number. Vaishnavi from Lord Vishnu, Brahmani from Lord Brahma, Indrani from Lord Indra, Kaumari from Lord Karthikeyan, Maheshwari from Lord Shiva, Varahi from Lord Varaha, Narasimhi or Pratingira from Lord Narasimha.
While the above are accepted forms under the present context, the manifestation of Goddess Yogishwari was mentioned in the Varaha Puranas.
Why the Yagya: The objective of performing Ashta Matrika Maha Yagya is for invoking the divine power of the eight goddesses through the fire ritual. This Maha havan goes on for two days the first day is allotted for Japa while the next for homam in the name of the eight goddesses.
Benefits and Significance: The main and common benefit of performing Ashta Matrika mahayagya is that the devotees get blessings from all the eight divine goddesses and it also fulfils multiple needs in life, to overcome struggles and keep negative energies at bay.