Asitang Bhairav is one the Ashta swaroop of Bhagwan Kaal Bhairav who himself is the Rudra Avatar of bhagwan shiv.
Kaal Bhairav Avatar is the ferocious form of bhagwan Shiv which was taken by Mahadev during
the time when Lord Shiva became angry with Brahma Ji as one of his 5 heads used to lie.
Once there was a conflict between bhagwan Shri Narayan and Brahma Ji as to who is more superior. Bhagwan Shiv appeared in the form of Divya Jyoti (divine light) and told both that one who finds the extreme end of the light will be considered more superior.
Lord Brahma sat on a swan and went above whereas Bhagwa Vishnu sat on a wild boar and went down. Both the devatas after much persistence couldn’t find the end of the Divya Jyoti.
Lord Vishnu conceded that he couldn’t find the end whereas Bhagwan Brahma lied and said that he did manage to find an end hence he is more superior. This made Bhagwan Shiv very angry and he took the swaroop of Kaal bhairav to cut the fifth head of bhagwan Brahma.
Bhagwan Kaal Bhairav attained to Brahma Hatya Dosha as the head of bhagwan Brahma remained stuck to his hands. Shri Hari Narayan told Kaal Bhairav Ji to go to Kashi to get rid of the dosha.
Just as when Kaal Bhairav entered Kashi the head fell to ground and the Brahma Hatya Dosh was done away with. The place where the head of bhagwan Brahma fell came to be known as ‘Kapal Mochan Tirth’.
When bhagwan Kashi Vishwanath was returning to Kailash he handed over the responsibility of Kashi in the hands of Bhagwan Kaal Bhairav. The Ashta swaroop of Kaal Bhairav guards the city of Kashi from all the 8 directions.