Skanda Purana 4.101.33

4.101.33

31 brahmaṇaḥ śiraśchettuṃ śivātkālarājasyotpattiḥ, śivīniṃdākāribrahmaśiraśchedātkālarājasya kāpālikaveṣeṇa kāśyāṃ prayāṇam, tatra gatasya kālabhairavasya pṛṣṭhalagnāyā brahmahatyāyāḥ pātālagamanam, tatra kapālamocanatīrthe sarveṣāṃ pātakakṣālanāya kālabhairavasthitivarṇanaṃ ca. -...... -.. -. -. -. 142 2

31. The birth of Kālarāja from Śiva in order to sever Brahmā's head; following the severance of Brahmā's head for reviling Śiva, Kālarāja departs for Kāśī in the guise of a Kāpālika; Kālabhairava arrives there, and the sin of slaying a brahmin adhering to his back descends to Pātāla; and description of the presence of Kālabhairava at the Kapālamocana tīrtha to wash away the sins of all. 142 2