In MahaShiv Puran on Narad’s request Brahmaji said-

“During the period of Mahapralaya (Final annihilation) nothing exist, except Shiva. The power which Shiva manifested from his own self came to be known as Ambika. This Ambika is understood to be the cause of the whole world as well as the Nature. Lord Shiva then created a Shivaloka, which is known as Kashi. It is the abode of Shiva and Parvati. A man who makes a pilgrimages of Kashi attains salvation.