Śiva-śakty-aikya-rūpiṇī शिव-शक्त्यैक्य-रूपिणी (999)

This is the most revered, admired, perplexed, incomprehensible and secretive form of the Divine Couple, the confluence of Śiva and Śaktī.  The universe is created, sustained, dissolved and re-created by them at their will that is indentured by the law of karma.
Saundarya Laharī (verse 1) says, “Śiva becomes capable of creating the universe, only when united with Śaktī, otherwise He is incapable of even a stir (known as spanda).” Śiva is also known as Parabrahman and Śaktī as ParāśaktīParabrahman is the static energy and is niṣkāma (devoid of  desire, disinterested and unselfish) in nature. The nature of Śiva is explained in Māṇḍūkya Upaniṣad (7), which says, “It is neither consciousness of what is happening within nor the consciousness of what is happening externally.  It is not conscious of all objects and it is not unconscious either. It is beyond perception of any organ, beyond thought and sound. In it there is only consciousness of the Self and there is a total cessation of the world as such. It is the embodiment of peace and all that is good. It is without a second.” This is the typical explanation of the Brahman that is preternatural and interpenetrating.
The explanation provided in Varivasyā Rahasya, the treatise on Pañcadaśī mantra, corroborates with the explanation offered for Kāma Kalā Vilasa, yet another treatise on Śrī vidyā. Varivasyā Rahasya (verses 69 to 72) says, “Śiva and Śaktī embrace each other. The Brahman (Śiva) with the desire to create glanced on His other half, His consort and assumed the form of a bindu (referring to male procreative fluids), into which Śaktī enters assuming the form of another bindu (representing female procreative fluids).  The mixture formed by their aggregation is known as aham or I, the ego.”  This is the subtle form of the union of Śiva and Śaktī.
One more interpretation is possible for this most secretive nāma of this Sahasranāma. The scene goes like this. Śiva is sitting alone meditating, His usual posture. Śaktī enters the place of Śiva. Śiva wakes up. First, Śaktī sits next to Śiva. Later on She moves to His left lap and finally occupies His entire left side, blessing the universe with their Ardhanārīśvarā form causing creation and sustenance. When Śaktī moves away from Śiva, He starts His cosmic dance, causing annihilation. Śaktī witnesses His cosmic dance (nāma 232 and 571).
Saundarya Laharī (verse 34) says, “I consider your pure frame to be Śiva.  Hence the relationship of the principal and the accessory exists in common among you both who as transcendent bliss and consciousness are equipoised.”
This nāma salutes Her undifferentiated form from Śiva, that is inseparable eternally. Their union is also known as Śiva-Śaktī sāmarasya, the identity of Consciousness, where identical state prevails in which all differentiation has disappeared.