The pebble the myna dropped (Jalodbhava, Satisara and Hari Parbat)
Kashmir was the lake Satisara, and the demon Jalodbhava in it could not be killed in water. Kashyapa brings the gods to Naubandhana; Ananta splits the mountain with his plough, the lake drains, and Vishnu takes the demon's head with the discus. Srinagar's tradition adds that the Goddess, as a myna, dropped a pebble from Sumeru on the demon that grew into Hari Parbat, and Navreh is the day she settled there.
The valley is the lake Satisara; the Nagas live in it under Nila, Kashyapa's son.
Jalodbhava, son of Sangraha, is reared by the Nagas and wins from Brahma the boon that he cannot die in water.
He eats men and empties the Madra country and its neighbours.
Nila brings Kashyapa north on pilgrimage; they find the Madra country empty and Nila confesses.
Kashyapa and Nila go to Brahma's world; Brahma proposes Naubandhana; the gods come down to the three peaks.
The demon stays under water; Vishnu has Ananta split the Himalaya with his plough and the lake drains.
The demon makes darkness, Shiva holds up the sun and moon, Vishnu fights him and cuts off his head with the discus.
The drunk discus wanders, Shiva catches it and jokes with Vishnu; the peaks are named; Kashyapa settles the land, half the year to the Pishachas until Chandradeva learns the rites.
In the hill's telling the Goddess as a myna drops a pebble from Sumeru on the demon in the remaining pool; it grows into Hari Parbat and she stays as Sharika; that day is Navreh.
Nila tells Kashyapa that he reared Jalodbhava, son of Sangraha, who got boons from Brahma and has depopulated Madra, Gandhara and the lands of the Shakas and Khasas (141 to 146). Kashyapa goes to Brahma's world with Nila; Brahma proposes Naubandhana (147 to 152). Vishnu on Garuda, Shiva on the bull with Parvati, Brahma on the swan, Indra, the sages and the rivers gather; the demon, confident he cannot die in water, stays under it (152 to 168). Vishnu asks Ananta to break the Himalaya with his plough; the water rushes out; the demon makes darkness, Shiva holds the sun and moon, Vishnu fights the shape-changing demon and cuts off his head (171 to 180). The peaks are named and hermitages built; the blood-drunk discus is caught by Shiva, who jokes before returning it (181 to 200). The Wikipedia summary of the Purana adds the Nagas' refusal to share, the Pishachas' six months, and Chandradeva learning the rites from Nila.
After Kashyapa drained Satisar a remnant lake stayed near Srinagar with a demon in it (Jalodbhava; or the brothers Tsand and Mond, one in the water by the hill, one above the Dal gate). The people prayed to Durga, who took the form of a haer, a myna, flew to Sumeru, picked up a pebble in her beak and dropped it on the demon; the pebble grew into the hill and crushed him, and she made it her home. The hill is Hari (myna) Parbat or Sharika Parbat; she is worshipped as the eighteen-armed Sharika in a self-formed Shri Chakra on a rock on the western slope, reached by 108 steps from Deviangan. The day she took her seat is kept as Navreh, when Srinagar's Pandits climbed the hill with yellow rice.
What is different here:
The Goddess, not Vishnu, defeats the demon, with a pebble rather than the discus; the hill itself is the weapon; the story ends in a festival day.
Sources: The Nilamata Purana, English translation by Ved Kumari (1968), verses 101 to 200 (Wisdom Library); V. D. N. Rao, Essence of Nilamata Purana, chapter 4 (kamakoti.org); Nilamata Purana, Wikipedia; Chakrishor (Hari Parbat / Sharika Peeth), Chander M. Bhat, on ikashmir.net; Hari Parbat, ecoheritage (C.P.R. Environmental Education Centre); Hari Parbat, Wikipedia; Geeta Vaishnavi, Hari Parbat: a centuries-old celebration of diversity in Kashmir (JK Arts Foundation, 2022)