The tradition of such guhya or secretive Bhairava is undoubtedly an oral tradition that is continuous while in the guru disciple tradition. Their karmakand rituals and procedures are key, they aren't created down but only learnt and practiced in guru disciple custom. The Rigveda explained Rudra as the entity in https://directory-engine.com/listings13056524/considerations-to-know-about-kaal-bhairav