Maa Siddhidatri is the 9th form of Durga. She sits on a Lotus and rides on the lion. In her four hands she holds a Gada, Chakra, a Lotus flower and Shankh. To her devotees she bestows, according to the Markandaye Purana, the eight Siddhis: Anima, Mahima, Garima, Laghima, Prapti, Prakamya, Ishitva & Vashitva. Therefore she is worshipped by not only humans but also Devaghana, Gandharvas, Asuras, Yakshas and Siddhas.
It is writen in Devipuran that in the beginning of the universe Lord Rudra worshipped Adi-Parashakti for creation. It is believed that Adi-Parashakti had no form. Lord Shiva, Himself got all these Siddhis by worshiping Maha Shakti. By Grace Adi-Parashakti appeared as the form of Siddhidatri from the left half of his body. Thereby he is named the famous Ardhanarishvara. This form of Durga is worshiped by all gaining great achievements, felicity and perfection.
Maa Durga removes ignorance and provides the knowledge to realize the Divine. That is why She is surrounded by Siddhas, Gandharvas, Yakshas, Demons and Gods who are worshiping her. The Siddhi that she provides is the realization that everything is Supreme Being or Brahman. If a sadhaka worships in the prescribed manner with full faith he draws the Devi's Grace and acquires all Siddhis and nothing remains unattainable by him in the universe.
~*~ Jai Jai Maa! ~*~