Nemesis was born from Nyx, the Night, in the earliest generation of the Greek cosmos, before the Titans, before the Olympians, before the world had shape or law. In Hesiod's Theogony, the great poem of divine genealogy composed around 700 BCE, she is listed among the children of Night, alongside the Fates, the Furies, and Death himself, a family of implacable powers who govern the aspects of existence that no mortal can escape. Her name means "distribution" or "due allotment," and her original function was not merely to punish the wicked but to maintain the balance of fortune, ensuring that no mortal received too much happiness or too much suffering, that the proud were humbled and the humble were raised up. She was not evil. She was necessary, the divine mechanism that kept the cosmos from collapsing under the weight of human arrogance.

Her most famous myth is the chase, the pursuit by Zeus that would produce the egg of Helen. The story is told in its fullest form in the lost epic the Cypria, preserved only in fragments and summaries, and in the works of later poets and mythographers. Zeus desired Nemesis, not with the casual lust that drove him to mortal women but with a passion that bordered on obsession. He pursued her across the earth, and she fled before him, transforming herself into every creature she could imagine to escape his grasp. She became a fish and swam through rivers, but Zeus became an otter and followed. She became a bird and flew across the sky, but Zeus became an eagle and soared after her. She became a lioness and ran through the mountains, but Zeus became a lion and ran beside her. Through every transformation, the chase continued, until finally Nemesis, exhausted and cornered, became a goose and tried to hide among a flock on the shores of a lake. Zeus became a swan, the most beautiful bird in the world, and he mingled with her in that form, and from their union she laid an egg.

The egg was not ordinary. It was larger than any bird's egg, its shell marked with patterns that seemed to shift and change in the light. Nemesis, ashamed or unwilling to hatch it herself, left it in a marsh or gave it to Leda, the queen of Sparta, to care for. In some versions of the story, Leda found the egg and placed it in a chest, and when it hatched, Helen emerged, the child whose beauty would one day launch the thousand ships against Troy. In other versions, Nemesis herself hatched the egg and then gave the child to Leda to raise, so that Helen had two mothers: Nemesis, who gave her birth, and Leda, who gave her a throne. The double parentage was the Greeks' way of explaining how a single woman could be so beautiful that she seemed divine and so destructive that she seemed like a punishment sent by the gods.

Nemesis's role in the punishment of mortals was constant and terrifying. She pursued Narcissus, the youth whose beauty was so perfect that he fell in love with his own reflection in a pool and wasted away, because he had scorned the love of Echo, the nymph who could only repeat the words of others. She pursued Aura, the breeze goddess who had boasted that her virginity was purer than Artemis's, and drove her to madness and rape. She pursued every mortal who boasted that their wealth, their beauty, or their power made them equal to the gods, and her punishments were always precise, always proportional, and always inescapable. The Greeks did not pray to Nemesis for mercy, because mercy was not in her nature. They prayed to her for balance, for the hope that their own good fortune would not attract her attention.

The greatest monument to Nemesis stood at Rhamnous, a small town on the coast of Attica northeast of Athens. The sanctuary there was one of the oldest in Greece, and the temple housed a colossal statue of the goddess carved by Agoracritus, a pupil of Phidias, the most famous sculptor in Athenian history. The statue showed Nemesis winged and majestic, wearing a crown of golden deer and holding an apple branch in one hand and a phial of water from the river Styx in the other. The water of the Styx was the most terrible oath in the Greek world, the river by which the gods themselves swore, and its presence in Nemesis's hand signified that her judgments were as binding as the oaths of Olympus. The statue was said to have been carved from a block of Parian marble that the Persians had brought to Marathon in 490 BCE to make a trophy of their expected victory, and when the Greeks defeated the Persians at Marathon, the block was dedicated to Nemesis as a sign that the goddess had punished Persian hubris with Greek victory.