Devi Siddharatri is the 9th form of Devi Durga, but She was the first to appear at the beginning of Creation.
Lord Shiva contemplated on His Devi aspect at the beginning of the universe, as without Shakti, Shiva cannot create. Thus, this form of Devi, appeared from the left half of Shiva, and is popularly worshipped as the left part of Ardharnarishwara form of Shiva Shakti.
Siddhi means perfection. This form of Devi bestows on us all the material perfections and also the ultimate perfection of attaining Divinity.
Man becomes perfect only when He attains Divinity, irrespective of the material perfection that he may attain in his mundane life.
Devi Siddhidhathri is worshipped on the last and final night of Navaratri, as the bestower of all perfection both mundane and spiritual.
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