Vijñāna Bhairava Tantra 1.122

1.122

एवमेव दुर्निशायां कृष्णपक्षागमे चिरम् । तैमिरं भावयन्रूपं भैरवं रूपमेष्यति ॥१२२॥

evam eva durniśāyāṃ kṛṣṇapakṣāgame ciram | taimiraṃ bhāvayan rūpaṃ bhairavaṃ rūpam eṣyati

anuṣṭubh

— in a deep dark night ; — the dark form ; — the Bhairava-form

Just so, in the deep night at the onset of the dark fortnight, contemplating the dark form for a long time, one will reach the Bhairava-form.