Matsya Purana — The Attendant Hosts of the Sun and Moon: Monthly Gaṇas
एवमाप्यायते सोमः शुक्लपक्षेष्वहःक्रमात् ततो द्वितीयाप्रभृति बहुलस्य चतुर्दशी //
evamāpyāyate somaḥ śuklapakṣeṣvahaḥkramāt tato dvitīyāprabhṛti bahulasya caturdaśī //
Thus Soma (the Moon) is replenished day by day during the bright fortnight; thereafter, beginning from the second lunar day (tithi), he diminishes through the dark fortnight up to the fourteenth.
It does not describe Pralaya directly; it explains cyclical cosmic regulation—Soma’s waxing and waning—which is part of the Purana’s broader view of orderly time cycles that continue across creations.
By clarifying tithi-based lunar change, it supports correct observance of vratas, śrāddha, and monthly rites; a householder and a king are expected to sponsor and perform rituals at proper lunar timings.
The ritual takeaway is calendrical: many temple rites and vrata observances depend on śukla/kṛṣṇa pakṣa and specific tithis (e.g., caturdaśī), so knowing Soma’s waxing/waning guides correct scheduling.