On the ninth day of Navratri, Maa Siddhidatri is worshipped. Siddhadhatri literally means giver of siddhis (power, meditative ability).
Maa Siddhidatri is depicted with four hands, sitting on either a fully bloomed lotus or a lion, and holds a Gada (mace), Chakra (discus), a Shankh (conch shell) and a lotus in each of the four hands. She is worshipped by humans, gandharvas, asuras and devas alike.
As per the legend, Lord Shiva attained the eight siddhis only with the boon of Maa Siddhidatri. The eight siddhis attained by Lord Shiva are Anima (reducing one’s body to the size of atom), Mahima (expanding one’s body to an infinitely large size), Garima (becoming infinitely heavy), Laghima (becoming weightless), Prapti (omnipresence), Prakambya (achieving whatever one desires), Ishitva (possessing absolute lordship) and Vashitva (having the power to subjugate all). It is because of Maa Siddhidatri that Lord Shiva and Goddess Shakti came together as Ardhanareshwar.
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.