When Brahma Dev wanted to create life in this universe, He asked Shiva to separate His female element. Shiva and Adi Shakti got separated and creation of life began.
But Adi Shakti lost the Shiva element within her and due to which she couldn’t return back to Shiva.
Without Adi Shakti, Shiva felt dejected and lost interest in all the activities of world.
Necklace of Skulls
To keep Shiva engaged and attached with the world, Narayana and Brahma dev decided to bring Shakti back to Shiva. Since Shiva became a celibate human, Shakti needed to be born as human.
Since Shakti lost the Shiva element and His wisdom, She took birth as Sati many times but remained incomplete and inappropriate for Shiva.
For many births, Shakti kept on acquiring the human traits, emotions, attractions, desires, arrogance, stubbornness and materialistic attachments.
Due to this, Shakti needed to leave her body and took birth again and again. The skulls in the necklace of Shiva belonged to each birth of Shakti.
Because of the pain and separation, Shiva didn’t want Shakti to get back to Him but Narayana and Brahma Dev kept on trying, because Shiva was all alone without her.
Then Shakti took birth as Parvati, who did an austerity for about 3000 years. Within this time, Shiva tested her devotion and determination in number of ways and she passed in all.
Parvati was free from all the disorders. She married Shiva and learned Tantra Shastra from Him.