Matsya Purana — The Attendant Hosts of the Sun and Moon: Monthly Gaṇas
पितृभिः पीयमानायां पञ्चदश्यां तु वै कलाम् यावच्च क्षीयते तस्माद् भागः पञ्चदशस्तु सः //
pitṛbhiḥ pīyamānāyāṃ pañcadaśyāṃ tu vai kalām yāvacca kṣīyate tasmād bhāgaḥ pañcadaśastu saḥ //
As the Pitṛs partake of and ‘drink’ the Moon’s fifteenth digit (kalā) on the fifteenth day, it steadily wanes; therefore that portion is called the fifteenth share.
It does not describe Pralaya directly; it explains a ritual-cosmological cause for the Moon’s waning—Pitṛs consuming the lunar kalā—used to ground dharma practices like Śrāddha in cosmic order.
It supports the householder’s duty to perform Pitṛ-yajña/Śrāddha with correct lunar timing (tithi/kalā), presenting ancestor rites as harmonized with cosmic rhythms—an emphasis also upheld by kings as guardians of dharma.
The significance is ritual rather than architectural: it links lunar phases (especially the fifteenth kalā) to Pitṛ offerings, reinforcing why Śrāddha and related rites are scheduled by tithi and the Moon’s waxing/waning.