The Curse and the Bargain
It began with a curse. Indra, king of the gods, once slighted the sage Durvāsā, and the sage's anger drained the strength from the three worlds. Weakened, the devas lost battle after battle to the asuras, their old rivals. Desperate, they went to Viṣṇu, who gave them the strangest counsel in all the Purāṇas: make peace with your enemies, and churn the ocean of milk.
So the ancient enemies met on the shore of the Kṣīra Sāgara and struck a bargain. They would uproot Mount Mandara to use as a churning rod, coil the serpent Vāsuki around it as a rope, and pull — gods on one side, demons on the other — until the ocean surrendered the amṛta, the nectar of deathlessness, to be shared. Viṣṇu himself, in his tortoise form Kūrma, slid beneath the mountain to keep it from sinking into the deep.
The Poison First
They pulled for a thousand years, and the mountain ground and the serpent's breath scorched the asuras' faces. Then the ocean began to give. But what rose first was not treasure. It was halāhala — a poison so black and burning that the worlds themselves began to smoke.
Every hand dropped the rope. No one could hold it; no one would drink it. The gods turned to Śiva. The great god cupped the poison in his palm and drank it as if it were water, and held it in his throat so that it never reached his stomach — and his throat turned blue. This is why he is called Nīlakaṇṭha, the blue-throated one. The ocean, cooled, began to give its treasures.
The Fourteen Jewels
Out of the churned water rose the wonders, one after another: Kāmadhenu, the wish-fulfilling cow; Uchchaiḥśravas, the white horse; Airāvata, the white elephant that became Indra's mount; the Kaustubha gem; the pārijāta tree; the apsarās, dancers of heaven; Vāruṇī; the moon; and the goddess Lakṣmī herself, who chose Viṣṇu as her husband. At the last, the divine physician Dhanvantari rose from the waves holding a golden pot — the amṛta.
The demons seized the pot at once. Viṣṇu saw the worlds about to fall into the wrong hands, and appeared as Mohinī — an enchantress so beautiful that the asuras forgot to fight. She offered to divide the nectar fairly: the asuras on one side, the devas on the other. And as she poured, she gave every drop to the gods. One asura, Rāhu, slipped among the devas in disguise and tasted the nectar — and the sun and moon named him. Viṣṇu's discus cut him in two, but he had already become immortal: his head and body live on as Rāhu and Ketu, the shadow planets that swallow the sun and moon in every eclipse.
The tellings differ on details — the Purāṇas each churn the story in their own way — but they agree on the shape of it: out of struggle comes every good thing, and the poison comes first, and is drunk by the one who loves the world.