Bhairava is Shiva in his fierce and terrifying aspect, naked or smeared with ash, carrying a skull-topped staff (khatvanga), attended by a dog. The Puranas explain his origin as an act of divine violence and its expiation: when Shiva severs the fifth head of the creator Brahma (in punishment for Brahma's arrogance), the skull sticks fast to his hand, and Shiva must wander as a skull-bearing beggar (Kapali) until the sin of brahminicide is discharged at Varanasi.
In the Tantric and Kashmiri Shaiva traditions, however, Bhairava is not a subordinate or penitential form at all but the supreme reality itself, addressed directly as the absolute; the Vijnana Bhairava Tantra, a central text of Kashmir Shaivism, is cast as his teaching. The Vijnana Bhairava Tantra itself is difficult to date with precision.
Scholars generally place its core between roughly the sixth and eighth centuries CE, among the older strata of Kashmiri Shaiva scripture, and it survives chiefly through the extensive commentary the tenth century philosopher Abhinavagupta built around it in his wider synthesis of Trika Shaivism, so much of how the text is read today depends on his interpretation as much as the verses themselves. The Kapalika ascetics associated with the skull-bearing, penitential Bhairava are known largely through the polemical accounts of rival traditions and a handful of inscriptions, leaving their own practice only partially recoverable.